Yes please I'm always open to suggestions
Well, this is really great! Back in the day many of us hoped for some sort of Championship edition.
Before you get caught up in all of the extra stuff, then maybe just start with nailing the base game? Make sure it is the best possible (and closest) version of Final Fight.
One problem with all these extended levels is that it kinda makes the game worse in a sense. Let me explain. This isn't a game that has aged well because of the limited characters, lack of moves and monotonous enemies. However, it's easily remedied as long as you go the enhancement route.
Personally, I think the extended levels should represent a hard route of sorts? Maybe doors and alleyways lead to the extended levels, similar to SNK's Burning Fight or SORR. But I wouldn't do alternate routes. What I am saying is that maybe there is a vanilla route like the original game. But maybe for those who want to explore the rest of Metro city, then there are paths that take them there. The key to this is making sure they join up with the next point in the vanilla game. For instance, maybe in Level One you can go into one of those doors and kick some butt, then you come out to resume a level. Or you can take an alleyway before you descend into the basement, but then you return to the basement entrance after completing that part.
Even then, to justify this overly long game, variety is key. Since FF likes to repeat enemies with different colors (something ripped off from Double Dragon) then simply having different enemies to fight, and a good variety of them, shakes things up. Maybe taking the best new enemies from Final Fight 2 & 3 and incorporating them. There is a male skate punk that was used in the SNES version to replace Roxy that could be used here as her companion, for instance.
Unfortunately, since the sprites of many of these games are smaller, you would need a good sprite editor to help you make the artwork for both visually compatible. However, I see at least one person here who seems more than enthusiastic and up for the job.
Again, I really love this! There is some real potential here. Like Double Dragon Reloaded (which is the best example IMAO of how an openbor game can be just as good, if not better, than the the original).
If you are open to this dialogue then I can be more specific when offering my feedback.
Take care!
P.S. Also, love that you have Don Vecta helping you with this. Have been following his work for a while. The Subway extending opening is a great example of how to expand a level without it dragging, etc. He has a good instinct for this, and skills to boot.