I think the MC's powers are particularly hard to imagine how they could develop into something stronger. He's dependent on what the writer gives him in a way that other characters' aren't. Which I know sounds weird, they're all dependent on what the writer lets them do, but hear me out. Most of the characters' powers are self-contained. They source only from themselves and how they've developed them. That makes it conceivable how they'd get stronger. They'll do what they're currently doing, but more powerful and with more finesse and maybe some more creative applications. It's easy to think of ways Alice could direct her growth, for example. Better barriers, more space-warping, golems like Danu and Aos have, etc.
But the MC's powers are based on something external to him. He has to work with what biology he's found to copy. Like, if he wants to improve his blades, he can't really develop that on his own. He doesn't have the fine control. He has to find something that has better blades than what he's working with, and if he can't, too bad for him. He doesn't get to have better blades. And it's not like we have a compendium of every monster in the setting and every potentially useful biological trait they possess, so the MC's potential growth is kind of a blank space. We don't know what he might someday be capable of, which since his foes have well-defined feats of power to compare him to, makes it hard to think he'd ever reach them.
