Some points need to be covered about this topic. There's a lot of buzz that wants to ignore groundwork.
X-Men: Apocalypse getting a set date, doesn't mean it has a budget yet. For that, the initial reactions to X-Men: Days of Future Past in theaters needs to come back first. The budget for the next will more than likely be determined by that.
There are some time frame issues to take a look at as well.
If the movie takes place in 83 to coincide with the previous established pattern, we're looking at potentially a young Cyclops and Jean Grey that could be added, as well as a young Emma Silverfox if they decide to pick up the story pieces leftover from X-Men Origins: Wolverine and X-Men: First Class. This also opens the door to explain the Sabretooth character's evolution within the franchise. The biggest push here in general should be to making a coherent continuity that reflects the X-Men franchise as Star Wars does to it's own chapters.
In a post The Wolverine world, we have story points that also need to be covered like restoring Wolverine's claws and potentially starting to establish a replacement for his role through another character. Keeping a coherent cinematic universe that ages works in Fox's favor as it permits them to do a full box set release at any point. This can potentially continually bolster their sales. So in that kind of time period we have the door opened for X-23, among many others to start establishing a new modern X-Men team potentially led by Storm.
So the time frame issue dictates what characters are available to use outright.
Keeping the story in the past limits what part of the cast is available to use and the stories able to be told. It also fills in the gaps though to help make a better unified continuity. All the pieces are there, they only need a little polishing to fit into the overall timeline.
Using a future timeline again like the Age of Apocalypse (as per what the animated series Wolverine and The X-Men was going to do for season 2) presents many opportunities if they go that route. This being another altered timeline based on different events that are then corrected to coincide back to the previous timeline yields strong possibilities. This method allows them to tell a story closer to the original comic events as well. Having this movie be the counter point to X-Men: Days of Future Past where there is only a single character aware of the timeline change and that it needs to be changed back yields potential but at a higher budget. The question would be in how similar that would be to Days of Future Past though. This does open the door more for Bishop and Trevor Fitzroy as usages go, or potentially even Legion too per the original Age of Apocalypse story.
Until more details come out about where they are going with this, we are left with three potential open directions.
1.) 1974-1983, more likely 83 as it fits the 10-11 year pattern established between First Class and Days of Future Past. Though it also depends on major events per each year too. 1976 for example yields the nuclear tests in Nevada among many other historical events like the first female students at West Point Military Academy. After 1979, they can use a young Jean Grey and Cyclops in the cast as well as potentially wrap up a transition for the two Emmas to unify that story better and explain the mind hopping trick Xavier used in the original trilogy including habitation. It'd also explain the lack of Emma Frost in the original trilogy as it'd suit her story to stay away from Charles to prevent him from undoing her actions. After 79 also lets them better polish up what happened with Deadpool to make his story start to line up to what fans expect as the character(though X-Force can do this too via flashback). It also opens the door for more New Mutants cameos, as well as bringing Gambit into the story as well.
2.) After the events of The Wolverine gives us a Wolverine with bone claws that he may want changed back to metal, the potential for the Angel to Archangel transition, and opens the door to build up X-23 to eventually replace Wolverine's role in the film series once Hugh Jackman no longer feels he can continue in the role. By establishing Mr. Sinister in this era including the past, this also yields potential to bring a young Scott Summers and Jean Grey back and maybe line up to the comics some. In spirit it could start to go similar to where the All-New X-Men comic is starting to tread. With Mr. Sinister, it can also bring Gambit back into the story.
3.) An apocalyptic future world where the timeline was corrupted from the previous course-correction(or a technological mutant utopia where humans are enslaved as DOFP seems to already be post apocalyptic which helps serve as a counter point to DOFP). This yields an Age of Apocalypse world, but also gives us a Back to The Future parts 2 and 3 type scenario of a cliffhanger with Days of Future Past. This means that both past and future will collide again, but in a different manner depending on what event it is that kicked off the splintered timeline. Originally the comics had used Legion and his reality warping abilities, this time it could lean on Rachel Summers, or Nathan Summers, Franklin Richards, or any other time hopping characters like Trevor Fitzroy & Bishop. The timeline having been completely skewed by some point prior to X-Men: The Last Stand events yields the potential for characters that otherwise couldn't exist. Using Last Stand as the catalyst though, keeps them in place as deceased.
The only thing that seems certain for now, is it may feature the 4 horsemen that Apocalypse is known for. Each era dictates different potential and possibilities for this along with which characters that could or should be used. The other factor to consider is how much of this will still be a time travel story. Time travel as a story device is a fun one to use, but it's also one that may reach over-saturation by 2016. The X-Force film being released before this could alleviate that if time travel doesn't factor into their story. It having no set date as yet though suggests it won't be until after this film is released. It depends exactly how much of the events of Days of Future Past plays similarly to the serial franchise cliffhanger aspects that Back to The Future used.
Another possibility could arise after Days of Future Past. If the future X-Men team becomes audience favorites, that team may be where they lean heavier on while they continue on building the past alongside it. This means many of the characters featured will carry over to any events after Last Stand and allow for new versions of them with the same cast. This seems ideal as it lets them build both time periods concurrently while they have Hugh Jackman, Ian McKellan, and Patrick Stewart at their disposal. This also gives them the chance to start bolstering the presence of a new team to stand on their own footing while they continue on with the past buildup using Fassbender and McAvoy while they can. Ideally, building up X-23 in either X-Force or X-Men: Apocalypse or even as a focal point in the next Wolverine sequel also presents an opportunity to test the waters and even prep that character for her own eventual spin-off film series to take over Wolverine's slot.
With the next film announced, it only means that ideas are brewing. Nothing is final until a budget is set and the shooting script is being filmed. Even then, pick ups can be added to flesh out what's needed if changes are made as decisions are finalized. We may have a set date, but that doesn't mean everything about it is set in stone. The reactions and audience reception to Days of Future Past can still alter much of what is to come for X-Men: Apocalypse.
As for X-23 though, there's hope for her among all 3 upcoming films. She can be fit into X-Men: Apocalypse depending on the timeline usages, or she could be featured in the next Wolverine sequel, or she may even show up in X-Force. Who knows. She may even be a triple threat and appear in all 3. The character only needs to be exposed further to general audiences and shown the visual potential of her usages before they'll outright be clamoring for more. It worked with Marvel vs Capcom 3 on a smaller scale already as mild proof. With strong women becoming more the leaning point among audiences with established intellectual properties, it may be prudent for Fox to start to build her up for own origin spin-off with a moderately easier to manage budget that allows for higher returns. Black Widow is a fan favorite from the Avengers films but she isn't due for a spin-off any time soon. This leaves an open void for a Marvel femme fatale to fill that with the right casting and platform may generate sizable buzz and returns. It seems WB and Sony are already aware of this themselves. With Wonder Woman set to be in the upcoming Superman/Batman film, and Felicia Hardy being established in Amazing Spider-man 2, it's only a matter of time before each gets their own solo films. Black Cat is fan favorite among Marvel fans, and Wonder Woman as a solo outing can start to establish a larger world that includes Donna Troy, Artemis, and many more. It's probably time for Fox to start to consider the same before the pieces and timing are no longer in place and they have to play catch up to even the Marvel Cinematic Universe itself on this front.
We're in an age that wants a recognizable strong female protagonist. Might be time to strike while the iron is hot. That seems to have been the only hindrance to previous attempts that hit critical acclaim like the Saiorse Ronan film Hanna (leading Ronan to the also much highly acclaimed How I Live Now.) Reactions were overall pretty positive behind Haywire and Columbiana as femme fatales went. It seems the only thing holding all three back were brand recognition that being part of the X-Men or Marvel universe yields. Casting the right star also brings character exposure and would potentially cement a decent box office return as well as home digital/bluray/dvd sales. This might be one of the rare instances where pulling in someone that can brilliantly portray the character and all that she stands for but also already has her own strong following could bolster the overall appeal to general audiences. It could make a huge difference.
Casting Selena Gomez in the role of X-23 for example would generate as much word of mouth and buzz if not more so as the talented Gal Gadot as Wonder Woman currently is. Gomez brings much to the table with having an already strong following and an intriguing public image that generates amazing potential and discourse across the spectrum. While there will always be negative reactions as we're seeing with Gadot's casting as Diana, like Gadot, Gomez brings a brilliance to the role that only a select few others could possibly match. The others though don't bring anywhere near the amount of exposure she does to the character. The larger the reaction, as we've seen in the past, yields interesting box office results as it creates a universal curiosity into how the final product is.
It's a bright future for Marvel fans that want to see their favorite characters in cinemas, and DC is growing stronger in that presence too with each passing year. Only time will tell how this pans out. With each company striving to start paving the way for these vehicles and establish full on growing worlds with their own continuity and history, it may be high time for Fox to start doing the same using character-centric pieces and stories to do so. The X-Franchise is about team and family dynamics. Wolverine is about the character. X-23's story contains elements of both that can cater to all audiences and even pull in a wider audience spectrum. So it might be time to start grooming her to replace Wolverine as the solo outing for the day Hugh Jackman decides to retire from his role as Logan before it's too late.