4v6 is stupid as hell.
There are a lot of reasons why I shot this idea down without posting why, but since you remain adamant about it, here are those reasons.
The more skilled team, rolling with a standard 4 man team (
in this format), suddenly has to deal with 2 extra opponents. This is a huge deal, because the less skilled team can utilize very simple strategies that are difficult to stop, even with 1 class limit in place.
Consider the following. Each problem with all the extra classes that the 6 man team can choose from amounts to even larger problems for the 4 man team.
Constructor is there to defend at all times, or even push out to mid if he's ballsy. Good luck to the 4 man team pushing past 5 guys at mid, AND dealing with static defense + respawning players. Even worse, is if they lost anyone on the way, the offensive momentum can be easily stopped by the respawning players. No amount of skill can compensate for mass amounts of people hitting you.
The firebug is just another body to deal extra dps on offense and defense, especially on the latter where you can tag their healer with afterburn. They can easily hide behind the
meatshield, and pop out to deal damage. You don't even have to be a good player to play firebug in this manner, but you'll still get a lot of random kills.
A crap offensive deto can be a problem to the 4 man team. Even one explosion on a healer in the backrow makes them worth their weight.
Hide in the back and get random hitscan shots. Turns the tide of any
battle. You don't have to be a good rifleman to be a problem to the healer. Even if you aren't directly hitting the healer, you're spreading damage that threatens to stall their push. Probably the least worrisome of all the extra classes, because below average rifleman = never get good shots anyways.
The biggest imbalance of the idea, and without a doubt the one class that bad players will flock towards given the extra two slots. Jello spy is very difficult to stop because it forces the 4 man team to commit at least one person on backcap patrol. If they roll out with a 4 man push, it doesn't matter how awesome they are, because all it takes is for one spy to touch the intel and walk out of the base, and against 6 players, it's a guaranteed cap. If you commit 1 person on defense, the 4 man team now has to run a 3 man offense against a 4-5 man offense.
Therefore, the threat of backcaps, which I would imagine to be rampant in an imbalanced team game, does one of two things. It either forces you to commit fully with all resources to an attack and risk backcaps, OR you commit somewhat to defense, and your push will get rolled because you are vastly outnumbered.
It's a fucking ridiculous idea, and I don't think any amount of skill that the 'pros' have can compensate for the amount of bullshit that they have to fight through. GG2 is a LOT different than TF2. In TF2, skilled players have a significant advantage at aiming in 3d environments; soldier/demoman airshots, quick sniper kills, scout run and gun, etc. A pro team can roll through a bad one very easily. In GG2, offensive engagements are a lot slower and more deliberate. It usually comes down to healer-centric pushes clashing, and overweight + rocketmen shooting each other. The outcome is decided by a few well-placed, but not at all hard to do, shots from the supporting classes. So, the more supporting classes you have, the more likely you are to win engagements. Aim plays quite a small role, when you're backed up with 3 other guys.