The question is, what changes are suitable for the game for casual players and what's suitable for competitive players? Also, who gets to decide lol?
IMO, it's up to the casual and competitive players to decide for themselves how they want to play, by consensus. Over time, a new element/option/scheme is either generally embraced by a community or not and anyone can choose to get involved or not.
For me, custom terrains aren't like adding a new shape to Tetris. Custom maps and variety in terrain shapes are as old as the game itself, and everything else in my opinion (scheme options, rules etc) sits around that assuming the terrain works well for the scheme and there is consensus with the players.
You've commented before about a custom terrain on here, saying: "This map does it for me, it's time to use custom terrains for me now." Perhaps you never meant competitively? But I honestly don't know why you wouldn't want to apply your existing skills on new terrains competitively. You might enjoy it.