i don't think it's unreasonable to make suggestions or requests before the game is complete, that kind of defeats the purpose of the early access and these forums in general.
that doesn't mean they will, or even should/can, do everything everyone asks about, but the input may still be valuable.
i think the suggestion is potentially interesting, but i think i'd much rather it be in a separate mode or something outside the main game, because there's already so much stuff to do in the game as it is.
OP could also check out Sanctum 2 if they want more tower defense mechanics. It's an amazing game made by the same devs.