Malamber
What about pokemon and digimon transformations? And if the knowledge from another world's species works can't any imagined species work? What about ben 10 aliens, marvel mutants, inhumans?
What about becoming azmuth's race they are super smart and upgrade's race they are very useful? Can he become tensura's slime and get the MC's powers? What about monsters from there like milim's race? Can he become different kinds of slimes because in many worlds slimes have different powers? What about become a symbiote? Will he get different powers if he chooses different persons from the same race? Like different demons from the seven deadly sins?
Malamber:Off hand i would say Mutants and Inhumans would be a no go because at their base that are still human and his human form is Gar. Ben 10 aliens would probably be redundant because of all the alien life in this universe. Like I have said before Imagination will be a major key in the later transformations but the MC hasnt watched every bit of Anime or cartoon out there. Sorry to say but his imagination will be limited to mine with help of my readers.