A couple ideas seem obvious:
Lasting constructs. There's a pretty strong parallel between metal generation and stabilizing objects, both are abilities that Ella tried to teach the MC but that he didn't have the power to make practical at his current level. It could easily be taken further and have enduring creations be an ability that the MC makes use of once he evolves to Level 4, like how metal became a standard of his powers once he hit Level 2. And it would open up cool new options like living projectiles or bio-drones.
Electricity resistance. The MC's weakness to electricity hasn't been too major yet, he's been lucky to fight enemies who have to wield it through tools that he can disarm, but between Met and Indra it would be good for him to get it at least to the level where it's not a potential instant-kill. Given that his "exploding" is actually rampant cell growth tearing him apart, if he got his response to electricity under control it might even turn it into a strength, him absorbing lightning attacks to heal himself.
More improvements to his armors, them becoming even more specialized for power, speed, or memory. There are a lot of ways this could go, it's really up to WeirdWorld's creativity. Maybe the heavy armor gets explosive pistons in its limbs. Maybe the the light armor grows organic jet engines for even faster flight. Maybe the medium armor has horns that work like antennas so it can read memories at range. Those are just examples, there's plenty of room in the possibility space.
Being able to reach the dream realm on his own without assistance from another power. I'm certain he'll get this at some point, though it might be something he unlocks while still at Level 3. There's just too much left to see and do there, ancient ruins to explore and moms to visit. And it would make his memory powers really scary if he could prepare for a mission against S.I.N. by scanning and implanting memories ahead of time through dreamwalking.
