Halo, as has already been mentioned, but there's no way you could do it in $50 Mil. I'd want no-name actors, and probably do something like the games did with Reach or ODST, and introduce a new character. Yes I love Master Chief as much as the next guy, but his life/story is cannon, there would be NO creative freedoms/liberties. However I'd love to see a trilogy or quadrilogy of Master Chief as well, it just wouldn't happen in one movie.
I'd love to do X-Men and DO IT RIGHT, following the Uncanny X-Men Comic series. It sickens me that Stan Lee, let Fox make those abortions. Again no way it'd happen for $50 Mil.
As for stuff that hasn't been done, I'd do a Clans Vs. Inner Sphere Mech Warrior live action Series, or maybe one biggish movie that would hopefully lead to more. It'd have to be Michale Bay blow everything up CGI style, so $50 Mil will be pretty limiting as well. According to Wiki, a project is "in the works," but is "in development Hell."
Dark Horse had some AMAZING graphic novels of the Alien series. There are a couple that were produced between Aliens and Alien 3, which would have made a MUCH better movie than Alien 3. I've thought about a re-imagining, pretend Alien 3 never happed and watch this movie with that mind set. There was also a series, where the "company" has raised and trained some of the Xenomorphs, they were cognizant, spoke English, and helped the Marines. That would make a decent premise for Aliens 5. Along those same lines there were several Aliens Vs. XXX Graphic Novels (where Aliens VS. Predator came from). Aliens Vs. Terminator was a pretty good read.
Sorry I just completely NERDED out on everyone.