With Skyrim, yeah, the developer gives them free reign to mod the shit out of it - they actually tried to monetize that (or Valve/Steam did, idk), but that backfired harshly.
With GTA and the original FFVII game though, not so much. I know GTA got the shit modded out of it too using texture replacements and reverse-engineering - to the point where they somehow hacked multiplayer gameplay into GTA 3 and co - and FFVII got the same treatment, but the difference with a modern game is that modern games are much more complicated in terms of code and whatnot. But, I'm sure they'll find a way if they're skilled and dedicated enough.
Things like texture, music, and maybe whole model replacements should definitely be possible. Save game edits too. And if the rumors about it being made in the Unreal 4 engine are true too.. for all I know it can be modded using the engine, too.