What potential projects could people accomplish with this code?
Ports to platforms of similar power, and maybe slightly less.
Ready extraction of assets for use in other ports/remakes.
Bug fixes
Tweaks to take advantage of increased power of today's computers; longer draw distance, higher frame rate/unlocked frame rates, widescreen tweaks (
https://www.wsgf.org/dr/simpsons-hit-run says most are more hacky than official), more NPCs, better graphics shaders (not sure what they would go in for here as it is a cartoon game but there are many effects), modern takes on controls, better camera.
Better AI
All manner of house rule type mods, extra modes, a complaint from some is that the game maybe does not have the open world feel that GTA manages most of the time where you can cruise doing whatever and this could far more readily create that.
Day-night cycle maybe, whether you want to make some kind of persistent world (I know cartoon resets when the credits roll) or just some kind of larger reset limit.
Using the engine for a whole other game (debatable whether you would want to; this is older code when far more freely available things exist and can be shared with everybody).
More assets included (possibly random NPC generator if city of clones is too much for you)
Easier to make new missions, map expansions and more, including beyond the limits of what even a determined modder might have been able to accomplish before (find replace all buffers with 10 times the space which is still a drop in the bucket compared to what your browser with a few hundred tabs is likely using, press compile sort of thing vs having to do all that as an assembly hacker which is a months long effort most likely).
Multiplayer, including to a more extreme degree than might have been possible on any existing ports -- 64 player persistent map maybe.
This in addition to being able to go over the code to see things the devs might have done back when, or bits that might be available from it.
Generally while game modding is a thing there are practical and more expected limits.
For the most part unless another game using the same engine gets modded you are left with what tools the devs provide you, what abilities the devs left in for modding potential (and it is something devs have to add or foresee, and they can just as readily make it really hard) and what limited things skilled hackers using skills rarely taught in schools any more might go through debugging and assembly code sessions to find. Source availability, whilst still being limited by the quality of the source*, is then keys to the kingdom put into the hands of possibly half bright kid that just managed to get hello world to compile on code::blocks (while there is an awful lot of arcane stuff in C family languages at the same time a lot of it is fairly obvious and even if you do something that would be a system grinder back 20 years ago will today need specialist tools to measure the difference) to regular coders and even those skilled ones that do know how to play with assembly coding** will probably have their productivity as it were increased tenfold if given source.
*though some projects in previous source releases do make an effort to bring the code into line with more ideal coding methods and modern concepts and this is often the first main thing after the initial play around.
**my usual links for PC assembly coding introductions, and frankly for other assembly too with the idea you learn the basics and move sideways, are art of assembly
https://www.plantation-productions.com/Webster/ and Gavin's guide
https://stuff.pypt.lt/ggt80x86a/asm1.htm to get an idea of what that entails.
Leaked code as opposed to freely available code you might see on the likes of
https://osgameclones.com/ means any projects will operate with the threat of a takedown hanging over their heads, and if you thought Nintendo was bad at C&D/DMCA then you have clearly not been paying attention to Fox (
https://www.dailydot.com/unclick/fox-firefly-etsy-jayne-hat-crackdown/ , and there was also the time they sued themselves but that was just funny) and Disney (you know why copyright is only starting to see new stuff enter the public domain, and it being from the 1920s,
https://gbatemp.net/threads/new-in-the-public-domain-for-2021.580143/ , rather than 1960s... generally held as being Disney the ones responsible). This can make recruitment harder, people more reluctant to play, possibly dissuade some coders*** from contributing (or force them to be anonymous), possibly see things have to happen behind closed doors and then released, and maybe released as a complicated patch or like downloading any other piece of warez.
***even if it is "ethics shmethics" then it might not be a great look for coders in some positions or career paths to be seen playing in such circles.