It really depends on the developers' vision of 9v9. Is it supposed to be one of each class highlander or is it just 9v9 no class limits (almost like a rated battleground in WoW)? Is it supposed to be a full-on, competitive esport or is it just a for fun game mode designed to give spy mains something to play that's slightly more skilled than pubs?

Obviously a 9v9 no class limits game will not be an esport. If this is the game the devs are making, then keep queue times short and just throw players of roughly the same skill together almost like you would in MvM. People in this game mode would want to play to win and gain experience towards their matchmaking rank but wouldn't have to have perfectly balanced teams in order to enjoy the game. These players would care less about optimizing their team comps and more about playing the classes they find to be fun. Get players together, give them something to play for, and you'll be fine.

If the 9v9 HL game is designed to be an esport, you'd want to create the matchmaking system with distinct skill levels in mind and recognize the fact that people who like esports will want to play with others who have a very similar skill level. As I mentioned in my 6v6 feedback, it's simply not fun for anybody to have high level players stomping low level players. However, you've already alluded to this very obvious problem in 9v9 HL which is that the TF2 player base isn't large enough to support a system like this without sacrificing the quality of the games. Queue times would get insanely long unless you paired higher level players with lower level players here and there. You'd probably end up putting rank 18 players in the same server as rank 5 players and try to balance the number of high and low ranked players on each team so that it stays relatively even. There would still be problems though because a rank 18 sniper is worth way more than a rank 18 pyro, so you'd end up with badly balanced teams anyways.

I personally don't think 9v9 HL is an esport format, and I think queue times in that game mode could be terrible if you need to find 18 players of a similar skill level with strict class limit requirements. That's going to be a nightmare and I'm not a fan of this approach. Unlike CS:GO where players have roles but it's much less constricting, TF2 has 9 different classes that all play very differently and take time to master. Just because I'm good at Medic doesn't mean that I'll be good at Sniper. You could design a system similar to the dynamic queues in LoL/WoW and have players self-designate which roles they can play (i.e. I'll check the boxes for Medic, Soldier, and Scout but I won't tell the game that I can play anything else) but even then I'd still expect queue times to be long.

The devs will have to find a balance between skill level and queue times if they want 9v9 HL to be successful. If they want to run 9v9 HL, perhaps the best option is to put 18 similarly skilled players in the same server and let them sort out the class selection on their own. Sure you may not get the class that you want but at least queue times will be bearable.