Rebuilding my game from scratch, I'm planning on creating a game which has the soulslike (mostly Sekiro-based) and bullethell genres combined.
Some general ideas:
2 different combat "modes" which change depending on the boss' attack:
"Soulslike" mode: Think of Sekiro's combat system for it, things like parrying strikes with your shield, jumping over leg sweeps and ground stomps, and "backhopping" away from downward attacks, while either using heavy or light attacks, depending on when the timing is right. This mode would challenge a player's strategic thinking and timing.
"Bullethell" mode: Main inspiration that comes to mind here is Realm of the Mad God, with higher reliance on "macro-dodging" over micro dodging (basically always moving around in bigger spaces, not just slightly tapping the movement keys as you'd do in Touhou as an example), with the reflect ability being a big focus, as you'll have to face a lot of patterns that, without it, would otherwise be impossible to dodge without taking damage. Attacking would also be inspired from RotMG. This would test a player's reaction times and positioning capabilities.
#Clyne: Soulslike meets Bullethell
velvet vigil
Here's an (older) video on how the transition between soulslike and bullethell modes would look like, soulslike having a more realistic looking atmosphere while bullethell focusing on minimalistic neon colors (along with a bit of bullethell action shown in the video)