PlingAlternative gamemodes have been played in the past, and most of the feedback have been negative, although a lot of people seemed to enjoy some of them. Examples are Gravelpit, which was a loved map for spectators, but a lot of players didn't enjoy it, and Standin (well I think it's a cool concept...). There is also Turbine pro (generally frowned upon, with some notable exceptions) and a variety of Koth maps, the latter which is the game mode I feel is a good fit for 6s and should be played more.
A problem with KOTH (taking product as an example) is that the maps are very 1-dimensional, unlike 5cp where teams can get very creative during pushes and hold setups, KOTH maps rely more on dm and very basic pushes and concepts in pushing.(4-man sacs, uber trades to delay point time, forward holding) and while 5CP is stale currently, if creative unlocks similar to battalion's backup are added to the game, teams would have more options to break stalemates besides sacrificing roamers and 5min sniper duels.