If we were to get a completely new power, I don't think it would be in this evolution. More likely the next one up. But if WeirdWorld has a hard time finding more ideas for the current set, it's still possible they slap something new on. But I think there's probably still enough to do with the "morphing other people" and "morphing less natural materials" for now.
Also I didn't think about it at the time but, maybe absorbing stuff to fuse it to us ? Like, a blade of a gun for example...
Actually it's not exactly set in stone. It seems to be a rule, but having someone with a power such as adaptation and evolution being able to break that glass ceiling would be absolutely in the realm of possibilities for a plot twist.
As for the Nico talk : she's absolutely out of the MC's league right now. No question. She could end him before he even understand what's happening. With her power, either there's a huge trick to it that he'd have to figure out before the fight, or he'd need to evolve and get a new power to either protect himself or disrupt her own.
But for now, if we go purely on a "what can they do regardless of level", she's a bad matchup for him, possibily the worst matchup, baring we get a superhuman with electric powers.