There is no combination of class limits, ruleset, gamemode or competitive platform that will make TF2 a successful competitive game without valve putting money into it, and even then the game may just be too old to be marketable. I don't see the game ever growing unless valve makes a 3rd team fortress game. Every attempt at creating a competitive gamemode (including/especially valve's own) has been a colossal failure from the perspective of retaining a large playerbase.
The game also has severe fundamental flaws that prevent it from being enjoyed by a large amount of the playerbase. Some of these flaws are compounded in the 6v6 gamemode we currently play, but many of them exist in all versions of TF2 and aren't fixable without major balance changes to the entire game.