My two biggest suggestions are:
1. Removed role-specific MMR.
This is 3 invisible queue buckets that, as near as I can tell, actually make matchmaking worse (through overfitting). This is one of those things that increases queue times without adding anything positive. Though, I do understand that a lot of people think having role-specific SR means something, I think they are dead wrong.
This seems like an obvious easy fix to me. And something they can easily check. I would bet dollars to donuts that the queue time improvements they saw in the 1-3-2 Experiment were driven by lack of proper matchmaking more than anything else.
2. Restructure the roster to Tank, Support (off-tank plus control), Damage and Healer. Similar to what they did with the HotS role rework.
This would give us 11 main/off-tank, 13 damage, and 7 healer (not counting Echo). If they were to add 1 Tank, 1 Support, 1 Damage, and 3 Healer in OW2, then we’d be at 12/15/10 (assume Echo stays Damage). Which would be a pretty good spot to release heroes of each role with equal frequency. And it wouldn’t mean waiting years for a fix, because it’s just a few reworks plus OW2. Just setting up OW2 properly could avoid this problem stretching on for years.
Here’s an example of what that might look like: