I don't think a lot of these are worthwhile to build a film around.
The heavy hitters like Omega Red and Shadow King - well, I think Shadow King, while a good villain, might be a little too out there for the grounded film universe (I know the series has embraced the more fantasy elements in recent movies, but we still haven't touched on anything like the Astral Plane yet), and Omega Red I think is more of a small scale villain or someone that should be in a Wolverine movie. And something like just showing the X-Men team dynamics, and the confrontations they face within themselves - well something like that might work in an ongoing comic or TV show, but nobody wants to go to a big budget X-Men film only to see the personal drama of a bunch of superheroes.
Shadow King and the Astral Plane could definitely be an interesting dynamic however, if done right. I also think that for anyone wanting to bring Cyclops and Jean back, the Astral Plane is a much better way of doing that than Days Of Future Past. Like I've said before, Days Of Future Past isn't to retcon things and undo events of previous stories. However, it could easily be explained that, with Jean being a telepath, all the people she "destroyed" as Phoenix were simply sent to the Astral Plane. There have already been stories involving travel between the "real" world and the Astral Plane, and bringing people back from the Astral Plane who were imprisoned there.
Hell, just writing that, I would infinitely prefer Singer ignoring the end credits scene from X-Men: The Last Stand and using the Astral Plane to explain why Xavier is back, than the "braindead twin brother" angle.
And writing all of this, an X-Men 5 that involves the Astral Plane doesn't sound all that bad actually.
Again, as long as it's well done.
I also wouldn't mind Genosha, but we're already getting our Sentinels storyline. I saw the Savage Land mentioned, and while a small part of me wouldn't mind seeing that, a larger part of me thinks it would be absurdly cheesy to have an X-Men: Jurassic Park movie. I kinda feel like intergalactic star battles, and dinosaurs, are the line when it comes to X-Men movies. I hope we don't ever see the Savage Land, Shi'Ar, Starjammers, or anything of the sort on film. Keep that with the comics, cartoons, and video games.