I think 5v5 is the bare minimum that Dominion should have simply based on the mechanic that capture and defend uses. But what about larger teams, is there plans for larger teamsize gametype?

Simple question, and I think it would only really work in Dominion, where everyone sticking together is not necessarily the best idea all the time. Requiring group splits and smaller and more frequent skirmishes, even more than one teamfight occurring on the map at a given time in different locations.