who said we should split the playerbase, current system where you get 5 same role mains in the same game is absurd
you don't see faker being told to adc all of a sudden now do you
I can't think of any way you could have one team use traditional draft options, and the other use team builder, without it being unbalanced. Unless you mean remove the old system entirely, which would be a fairly big change.
In the current system, people gained their ranks by being able to play multiple roles decently. Standings would shuffle around a lot if suddenly people were able to play only their best role in every game, and people who worked hard to have two or three good roles would drop while people who insisted on only playing two or three champion, all with within the same role, would move up. Because of this, you couldn't reasonably say it was the same ladder as the current system, because it doesn't take the same measurement of skill. It values specialization much more than versatility. Which is okay if thats what we'd rather have measured, but you have to make it clear that you're now using a different set of measurements, or rather, a new ranked ladder.
Pick a role and for top/mid/jg AP/AD/fill damage types, so you don't end up entirely AD or AP. As for lack of role diversity you say that as though there is some at present.
This is a very short explanation. Can you elaborate? It seems easy enough to check off the lanes, but how do you check off roles in a way that every player is okay with the set up ahead of time? Are you suggesting every player ticks off their ideal mix of tanks, bruisers, assassins, utility, etc? And then the team builder finds and matches people within that desired pairing of five roles? Because I'm not sure if the system is currently robust enough to handle all that in a timely fashion.