I don't know about A/D - Using learned knowledge of the past, leagues have historically edged out A/D maps like Gravel Pit from the map pool and whilst I personally think Steel is a fantastic map and it'd be interesting to play from a 6's point of view, I can't imagine more than 5% of the community agree with me, so I think the format will be edged out of the 6's game mode.
For Koth though, we've gradually influxed one map into the pool, Viaduct and it hasn't proven completely unpopular and as the OP rightly points out, 5cp is a burden on stalemates and more focus should be to invest in using more Koth maps to add more variety to help universal appeal of the gamemode but also help reduce stalemates which is a big issue with the game in it's current format.
Koth_Bagel has had a lot of praise and should be considered.
Ashville / Coalplant has been used previously but phased out, maps like Lakeside have not been considered probably due to the strength of the sniper, maps like Warmtic to my knowledge have never been tested and I am not sure why, it seems like it MIGHT be viable for testing. Other than this, from the existing pool of maps, I can't see many other options unless you're a fan of Koth Badlands.
Going forward I would say it would be crazy not to introduce more Koth into competitive TF2, particularly 6v6.