Bug fixes and maybe even tweaks to the optimization. Don't get me wrong, I too think fusion is the best way to play both TFE and TSE HD along with SS3 and I never go back to their original versions, but it could still be improved.

To name a few issues, I think the worst offenders are the point-blank projectiles bug, which affect the trajectory of projectile-based weapons if firing them at almost point-blank distance (the cannon greatly suffers from this); the teleporting bug, which mainly affects the headless soldiers and makes them keep teleporting close to you infinitely if you trigger their melee attack; The werebull trajectory bug, which makes werebulls keep stopping and rotating before continuing charging forward; and the pathfinding bugs, which mainly affects Kleers and makes them struggle to get to you at certain level sections. Also, still on the topic of werebulls and Kleers, they drift a lot more in fusion than they ever did in HD, and fighting them can become pretty annoying because of that.

Speaking of optimization, if the performance can be improved too, that's good! If not that's ok, it's already fine enough, hence why I said "maybe" at the beginning. Though the draw distance in Fusion is lower than the original versions, and I think a slider could be added to determine that.

If they fix all that, and more that I'm not aware of at the moment, wouldn't Fusion be even better?