What if we used promo games as gatekeepers.
Riot did this when they introduced them, but now with DQ or as some people call it dynamic boost there may actually be a reason beyond just being frustrating.
What if you had to solo queue to complete your promo games. Perhaps you could still queue as DQ while in promo, but only your mmr and w/l would be affected but your rank wouldn't actually go up until you completed your promos solo. will this keep plats out of diamond? Maybe not but would probably keep bronze out of gold, gold out of diamond ect.
This does not solve the premade in game griefs but could be a step to a playable DQ.