I have no idea what's going to happen with the new MMR system, but assuming a lot of people end up queuing as a party of 4-5, I think that MMR should be split from the 1-2-3 bracket and the 4-5 bracket.

That way people can still party up and improve their MMR, but you won't have people going up against 5 stacks all day who are just grinding out MMR.

Could also have it try to make the matches even, in that a party of 4 will go up against another party of 4, or a 2/3 party will go against a 2/3 party?

Now you just need to fix the drafting aspect so it's more like captain's mode in normal ranked games, since drafting is like 90% of winning a game right now, and there's nothing you can do to proactively ban a potential problem hero like alchemist, brood, or meepo who can easily snowball and end a game quickly.