I always forget to use prosthetic tools during a boss fight, and since I use ichimonji double, I always finish with 16-20 spirit emblems

Also, it's pretty redundant that we still have access to older versions of prosthetics. It makes sense for the spear & umbrella, since they have unique executions/effects that can't be easily stacked. Easy fix for that is this. Keep Suzaku & Lilac separate, unless it's OP to merge them. As for the spear, instead of relying on full charge, a single press would be the normal thrust, press & hold the analog forward for spiral thrust, and press & hold back on the analog to use the cleave attack. As for the kunai, it would be a separate tool from the shuriken as well.

The tools we got now have always been good, but I wish FS added a few more ideas into them. A different move for aerial prosthetics (& combat arts too) would have been great.

And just throwing my idea in, prosthetic combos. There could have been a mist raven shuriken combo, where you have to equip gouging top & mist raven. Normal throws are just regular shuriken, about as strong as gouging top, but charged attacks cost 2SP, & will throw the shuriken close to the boss, which you can teleport to. On the other hand, if you use mist raven and follow up, you can use shuriken which will deal twice the damage, then follow up with chasing slice as usual. So basically Mist Raven will power up any offensive tool you use after.