All tanks should have a team-up with another other tank.

If they want to encourage 2 tanks, every single tank should have a teamup with another tank, and not just a stat boost. The only 2 are hulk/strange and peni/venom.

Magneto could use caps shield/thors hammer whenever they die. Groot could "upgrade" mjolnir to the axe. Thing and hulk could get a fancy knockback punch.

I think some extra fun factor for playing 2 tanks would go a long way.