Even without disputing whether Possum was intended to have more differences from JJ2 than textures, that forum hasn't been called Possum Forest for many years. The title is "Further Game Development." Other games have also hung out there, from Mechaius to M.I.A. to Openjazz, not all of them even strictly fangames. This is a topic about game development; it is exactly the forum you are looking for.

Since you attempt to volunteer me to lead your project, I will say that after MLLE, my plan is to retool its rendering code into a .j2l-running game engine, with basic code for objects, mask detection, and so forth. That basic engine will be able to branch off in different directions (different games), and MLLE can be expanded as needed to support new versions of the .j2l format required for those different directions (e.g. alternate numbers or orders of layers, level scripts, and so forth). Most ambitiously, I want to implement a game with externally stored objects and characters, so instead of just downloading additional levels, for instance, you can also download additional characters and use them to play through existing levels. But MLLE isn't done yet, so that's a while down the line. If someone wants to help with that somehow (e.g. writing a better program than Jazz Sprite Dynamite), I might be open to that, but otherwise, unless one of you has a really really great idea, I'm almost certainly not going to jump ship and be the main coder for some other game design entirely.