IMHO, 10man mythic progression was the best time I've had raiding in WoW. The culling of that (including the removal of flying / adding of pathfinder) are what killed retail for me. I'm not going to go into the merits / pitfalls of 10man v/s 20/5man mythic, as those arguments have been discussed here ad nauseum, just the simple point that this is important to me, and potentially others like me.
My suggestion, doubtfully new/unique, is to tie 10man mythic progression to the unlock of X-Realm Mythic. In this way it doesn't compete with the WF race and maintains 20man as the "king" in that regard. There would most likely still be a 10man WF race whenever it unlocks, but presumably less relevant as the ones that have completed the primary WF race would just be able to stomp it by then. What this does achieve is that it allows 10+ person guilds to be a thing and have a valid avenue to progress via raids.
Side Note: The simplest solution to class mechanic constraints from downsizing to 10mans (where not every class/spec can be represented equally), is to borrow the mechanic from Instructor Razuvious, where if you don't have priests to reliably MC the adds, there are mechanical controls available that allow for that. It doesn't always have to be exactly like this, but the same idea can be reused for every class/spec specific mechanic that needs to be ported to 10mans.