Title says it all!
It would be cool I wish
It would never work to have brutes playable in multiplayer.The core of Halo multiplayer is that every player starts off exactly the same at the start of each match. No player has anything that makes them more powerful than anybody else in the game. Elites work because they are, for the most part, just a re-skinned human and it’s purely cosmetic. So there’s no distinct advantage or disadvantage to playing as an elite.
Now, if brutes were playable, it would totally break that. Brutes are at least a head taller than the master chief and twice as wide, so present a much larger target. They also move slower and take 3 - 4 times as much damage as a spartan. So, like I said before, this would break the entire system where everybody starts on an even playing field.
On top of that, they were not playable in any of the original games, therefore they will not be playable here as they are not actually adding anything new to the game.