SS2 Complete Overhaul: Legal?

[6750b1c4dadd8Cortex Reaver]

I am thinking about completely overhauling System Shock 2 to create an entirely different game. So long as I release it as a mod to SS2, it's legal, right?

[6750b1c4db038Kolya]

I'm not a lawyer but generally, as long as people still need the original game, there shouldn't be a problem.

I would like to add that there have been many projects like that. None survived. And that wasn't because they were shut down. Overhauling the whole game is not a job for one person, not even if you are extremely gifted and had worked on other projects like that before. You need to have done that or have something really good to show for your new project to get other skilled workers on your team. Because we all had the idea of making SS2 newer and better at some point, that's nothing special.
I suggest you start by checking out and possibly helping with the Citadel Conversion Project.

[6750b1c4db14eCortex Reaver]

There's just a lot of argument on the topic of what you can and can't do, and I didn't really want to make waves. I have most of the maps for the game done, actually. I was going the "new-game-that-requires-Doom" route, but with SS2 instead, as the Dark engine allows a lot more customization.

[6750b1c4db255Kolya]

It's just that so many projects died before, especially from people working alone, but I don't mean to discourage you.
I wish you all the best and hopefully I can play your game some time.

[6750b1c4db528ZylonBane]

I am thinking about completely overhauling System Shock 2 to create an entirely different game. So long as I release it as a mod to SS2, it's legal, right?

What you're describing is called a "total conversion". There are only legal problems with these if the subject matter has an aggressive copyright holder (see "Alien Doom").