It would be easy to say "same but more!" which would be great, because the core is great. Art is great, animations are clean. Hits are punchy. Music is fitting, not too repetitive.
But I would tighten and polish up the core even more first before adding anything so you know you have a perfect gameloop to work off of
One thing is the performance issues, do all instances have to follow player every step, or can they update their direction on an out-of-phase delay of maybe 5 frames or more? Maybe implement a system where if instance limit is reached it spawns bigger fires instead of a lot of small ones. Exp droplets seem to stick around, maybe they should despawn? I like the lighting system, is there a way to optimize it?
Next thing is the variety of the enemies. The fires are fine, it's a basic enemy, but the tree just seems to be the same thing with no mechanical differences. Maybe the tree having a significantly different attack or movement pattern would set it apart more from the basic enemy.
The difficulty also seems to ramp up too high for your own power to be able to match too early. This seems like it should be handled via stat adjustment first so that any additional weapons/abilities can be already based around a balanced system. Minor tweaks can go a long way here.
Probably the main thing I would say is there is nothing to shoot for on the map. It is mechanically barren other than the obstacles, so the strategy is just to make a big looping path for yourself with no goal. Maybe have some points of interest on the map that help you with your growth as a character or story progression?
All in all - Great start! I kept wanting to replay it, which means your game loop is pretty solid. Keep up the good work!