As much as I would like to have ways to prevent these unbalanced team comps in aram I doubt giving preset roles is a real solution and will just bring in a whole heap of problems. It's just going to create unfairness on so many levels.
Who decides which champions fit in which roles? For example tanks, how broad do you want to make that role pool? Are you only allowing the real tanks or do you want to include champions that tend to build a bit tanky but aren't really tanks? How many roles should exist? Its easy to come up with five roles already, for example; adc, tank, mage, assassin, and support. But there are also those odd champions that don't really fit any role, should they be put into one of those roles anyway? Will you split up the tank role into groups of more or less tanky champions or the various classifications of Riot like Vanguards or Wardens? What about the other roles, will they be split up? I ask because if you get more then five roles then you create a new problem, which is weird combinations of roles instead of weird combinations of champions. Just like certain combinations of champions we have now are better or worse, you'll get certain combination of roles that are better or worse and you'll be back to square one.
What about the good/bad aram champion ratio for each role? Let's be real about this, some champions just plain suck on aram, some champions are gods in aram, with everyone having a chance to get anything this isn't necessarily a "problem". But if you start to divide the champions up into their own groups and start to pick players who can only get champions from one of those groups, then you have to start thinking about which groups contain a higher percentage of better or worse champions in them. I don't know these numbers but I doubt they are perfectly evenly distributed, which means the system will kind of play favorites with players.
How is this going to affect trades and rerolls? Can you reroll out of your preset role? I would think not, otherwise what would be the point of the system. A player that only has adc's in his champion pool will be pretty fucked if he ends up with a non-adc from the free rotation, he has no way out of such champions by rerolling if he is stuck in a non-adc role, he would have to trade first to get out of the preset role if this is even possible. But due to rerolls being limited to your preset role, players will be far more hesitant to trade for anything because it will affect their future reroll. Players that want to reroll will not trade away for a champion that isn't of the role that they intent to reroll into.
But what decides who gets what preset role? Will it be random or will it favor people that have a lot of champions in a role? What is fair, what is fun? There is also a lesser problem where rerolls become choices instead. For example; if you only have two adc champions and you get assigned the adc role, your reroll is no longer chance, it becomes a choice.
I got no answers or solutions to any of this, I just see a whole heap of problems, sorry man.
But, what you can do to help mitigate the problem of sucky team comps is ask for Riot to make 2 reroll the standard for every game. If I may plug a thing I've asked a few times before. Instead of gaining reroll points that eventually turn into rerolls that we have to hoard and are afraid to use because they are so precious and rare, just make two reroll per game the standard and do away with the saving up of reroll points. Everybody simply gets 2 rerolls every match, now that they are no longer lost in sequential matches we can actually use them to fix crappy team comps in every match. Will this completely solve the problem? Of course not, but it will help mitigate it, without all the troubles of preset roles.
