Its a no. The current framework of the couldn't support it, and the release style could quickly throw things out of whack for a few different reasons.
The thing is though, I always spit out 25 as the goal. It's a reasonable and achievable number, but I will do monsters as long as the game doesn't feel over crowded.
Separate subspecies/related species does work though.
yak, mini cow, bison, Ox, buffalo, etc
sea slime, river slime, lake slime, lava slime, muck slime, etc
bleeding mushroom, clamshell mushroom, button mushroom, morel mushroom, oyster mushroom, etc
That's 15 new monsters based of the original 3.