Ranked specifically already has the majority of what we would wanna see in custom games.
Ranked FFA is already a customs game mode, it should be a ranked playlist (Call it Lone Wolves)
Ranked Attrition is already a custom game mode, it should be a ranked playlist (Call it Attrition)
Ranked CtF, KOTH, Oddball, Strongholds is already a playlist (Call it Arena)
Ranked Slayer is already a mode, give it its own playlist (Call it Team Slayer)
Ranked 2v2 is as simple as reusing the Team Slayer mode, limiting the players to 2 per team and the score to 25 (Call it Doubles).
A Monthly Rotating Ranked Playlist could also be in place and could rotate the following modes (plus more in the future): Team Snipers, FFA Snipers, Shotty and Snipers, 1v1 Slayer, Swords Only, etc.
That would make ranked SO much more diverse and give people more variety in what they want to play. And from the perspective of someone who knows very little about game dev obviously I have no real idea of how difficult it would be BUT relative to the other stuff that needs adding to the game it seems like it would be one of the easier things to do sooner than the rest.