1. Create unique maps that are exclusive to 10man groups; that will make epic hard mode raids again for them; you can't mix them up with tiny and dominated-by-metas 5man or big and hard-to-form 20man because it would contaminate the hard mode race as it was with 25 vs 10.
2. Drop all notion of vertical progression on hard modes; nobody truly cares if someone geared better but they actually care about who is the better skilled; it's painful to play just to gear and it's not fun for almost anyone on mythic raiding or mythic+ higher tiers.
3. Stop overusing 5man so much; it has its place because it's small but for the same reason it's the most heavily affected by metas of a handful of specs because it has the least room for more specs; you really have to offer hard modes between 5 and 20.