I was going to talk about how good gundam build fighters is and how people should make more fics about it, but i remember now that Ant-Man exists. So how about this?
Put the main character in a crossover world with marvel and GBF, Make him related to both Plavsky particles and Pym particles(Born in Aria but gettimg accidentally teleported to Earth, then getting adopted by Hanp Pym or something) Then let him utilize both to his advantage, by controlling the gundam with plavsky particles and the size with pym particles.
This doesn't really make any sense, considering gunplas are small and dont have much weight, but i don't even think they follow this rule in the Ant-man movie, so... case closed?