So in the last issue of Avengers Arena, Reptil kept Hazmat from blowing everyone up (not intentionally, she was bleeding radiation) by turning into a giant crocodile, scooping her up in his mouth, and diving into the water. At first glance, I thought that killed him, while Hazmat eventually made her way back to the shore, but there's a panel at the end of some SHIELD guys hauling a large reptilian body out of the water, so I'll assume Reptil survived.
Normally Reptil's powers are the ability to change into all sorts of prehistoric creatures because of some magic thing in his chest. I thought it might be interesting if the radiation changes things so that now he transforms into radiation-enhanced types of those creatures. By which I mean Godzilla and his ilk. Just have him constantly turning into '50s style sci-fi horror monsters, all those critters that played on the fears the populace had about atomic warfare.
I know, we're past that now. It's all about genetic engineering and tampering gone wild. But if the writer was really concerned that readers would object to it being too silly*, there's an easy out. The magic amulet thing. It's magic, it might cause him to react differently than otherwise expected. Christos Gage already did that bit where Reptil was aged into a future version of himself, than managed to resist being aged back afterward. Some of that was his own desires, but the magic could be part of it. Or say the radiation made him think of those old films, and that's what prompts the shift.
* I don't know if that would be a problem. Radiation is still at the core of a lot of the '60s Marvel heroes' origins, but judging by the movies, people are trying to move away from that to things that sound more, plausible, I guess.