really like that mana is hard to come by, makes it feel like you cant cheese the game too easy, like that certain bosses require a different strategy.
there are two systems in the game I find uninteresting but have thought of some ideas that could be fun:

1. Current accessories are boring, my solution, monsters and bosses rarely will drop accessories(not too rare, maybe cap the max held in inventory to 1), they could do one of two cool things: Give you minor stats like a bee stinger that makes regular attacks poison on a low chance.  or Give you the access to some of the abilities the monster has or a more human version like orc gauntlets that give you a punch ability that stuns on a lower chance than the monster's ability. I think these would be fun to use, maybe add some more stat accessories too so that not all of your party would use the monster stuff but some would.
2. I have played through the game twice and have never used the monster infection abilities on an ally for anything other than personal interest, I think since the mana loss from them is permanent the buff  should be as well maybe make something special if happen if you stack too many/all of them on one character (probably would require some sort of nearly unobtainable  mana restoration item too accomplish)

the game is still a fun experience even with these two systems the way they are, just thought of them felt like sharing.