How's the PC modding scene for sonic adventure 1, 2, and Generations in terms of new levels, gameplay, not including all the bug fixes and unfucking the ports?
I've heard 4 is dogshit but i'll bite the bullet if its worth it.
I can speak for Sonic Adventure 1.
The modding scene seems fairly decent, but not super robust. There are some character mods that not only replace the characters in question, but even so as far as to attempt to retrofit them into the story with new cutscenes, such as this
Rouge mod or this
Cream mod. However, outside of the new cutscenes and character models, the game itself seems to be completely unchanged from the original, as you still play the orignal levels from Knuckles's and Tails's stories respectively.
However, there are a few mods I've tried that completely overhauled how the game plays. The first is this
Klonoa mod which takes you through Sonic's story mode with all the abilities of Klonoa, so instead of homing attacking everything, you have to pick things up with your ring and throw them around just like in Klonoa's games.
The other is the
Mario 64 mod, which as you might expect, sees you playing as Mario with all the abilities from Super Mario 64. It's quite difficult at times, since it's pretty obvious that the levels were scaled for Sonic's abilities and Mario is no-where near as agile as Sonic, but it did give me a chance to slow down and appreciate some of the finer details in the levels that otherwise would've gone unnoticed, such as some posters hanging around in the final section of Speed Highway.
As for new levels, I took a quick peek just now and found
this one that appears to be a custom level based on Hill Top Zone from Sonic 2, including an entirely new boss. Haven't tested it yet, but the comments make me feel pretty optimistic. There's also
this mod, which restores the first section of Windy Valley to that of a beta version. So it would appear that we're now starting to get new levels in the modding scene.
Hope that helps!