Ok so you mean to say - If you Gold player (main dps) - your duo Plat player(main tank)
Your main tank will swap from dps to tank and expect to have the same MMR mid game ?
This is again Open que 2.0 - we need role lock & role limit. At the least we need role lock on tank atleast 1 - not more than 2 in a game.
I dont want gold dps who is actually diamond tank mid game to swap to abuse the system.
This is easy to address. Only let me swap if I queue for both and then average out my rank based on the roles I queue for. Use that to determine the rank range of the match.
As long as you accept that people will throw the game if they do not get their preference. And no, you do not get suspended for it. Fake news.
Id be down for 2 minimum Tanks, 1 minimum DPS and 1 minimum Healing. 
This is kinda genius, you pick some number of heroes you prefer and the game matches you (as best as it can within a certain time constraint) with other people who pick different… and possibly even complementary heroes/hero pools.
Say I only choose to “prefer” 1 hero and it is Rein. The game will do it’s best to put someone else on my team who has “prefered” Lucio as one of their heroes. But it could take longer to get a game or there is a chance I won’t get a Lucio player at all because I wasn’t flexible in my choice, but maybe it will also try to place me with a Sym or Mei player. If those aren’t available within the time constraint, it goes to the next level which is I’m prefering Tank, after that it is I’m prefering brawl, etc.
Long term goals for sure, but they could start with something as simple as prefers different heroes so you don’t get 4 tracer players or 4 mercy players.