I don't see how you guys are just now complaining about 2 vs. 5s when that functionality has messed up GSing for the past X years
Limiting it to 5 vs. 5s would be bad for a bunch of reasons, I've posted what I think the best solution to that is a few times in the past (adjusting HPs across uneven teams)

In all honesty there is no perfect solution because at that point I could queue 2 vs. 5 with Sarah or someone like that and we'd have 7.5 HP each and it'd be ridiculous, anything you can think of that's reasonable is probably just as exploitable or harmful as the problem itself