... Isn't that what happens already? Different strategies obviously, but as far as I know, ANY competitive game will have X builds at high competitive. There WILL be builds/ships that are 100% ignored because they're useless in the meta. I don't play the high competitive stuff, but if they really see more than half the ships actively flown in most competitive matches (mind I'm saying most, because there is always the odd-balls that take something not full-meta but are good enough to make it effective), I'd be shocked.
You're correct in that there is always an outside bound (X) of how many different builds or strategies can be competitively viable at any given point. The idea, though, is to try and make X a higher number rather than a lower one. Diversity in play is a key indicator of a healthy competitive metagame.
One can look at past game modes that have negatively impacted play diversity as examples:
Skyball was a mode where speed is the only thing that matters. As a result, playing anything other than the fastest ship in the game (Squid at the time) was simply a disadvantage. Perhaps the most disliked game mode of all time. It was eventually broken in an update and removed.
Resource Race heavily emphasized map mobility, requiring teams to move around the map very quickly to obtain points. While not quite as stifling to ship selection as Skyball, you were limited to ships with strong long-distance charging speed and forward facing weapons (mainly Pyramidion and Goldfish, or Squid for non-combat strategy). Resource Race was also heavily disliked and eventually removed from the game.
(Crazy King is similar to Resource Race, but one step farther away from Skyball on the "bad scale".)
King of the Hill approaches the level of "modes that can be fun to play", but the forced close-range nature excludes otherwise viable builds due to every team composition requiring the capability to both contest and defend a single area of the map.
Stock deathmatch has always been the most balanced mode in the game. Pre-Alliance you had four to five viable ships at the competitive level (
2014 tier list,
2016 tier list) out of the original seven. Even now, in probably the worst state of strategic diversity of the last three years (a result of bringing over the poorly balanced Alliance ships to PvP) we still have roughly six competitively viable ships out of the available twelve.
Anyways, this is my (arguably solicited) analysis on how the mechanics of different game modes impact the diversity of viable ships.
As you've said: this "mode" can already be set up in the game client. So as a one-off event or perhaps part of the "Chaos Skirmish" rotation, it would probably be fine for the more casual audience who simply wants to experience new mechanics in a non-competitive environment.
We've had a lot of bad modes added to the game over the years. When I see posts for mode suggestions I can get overly vigilant.