Just sharing some thoughts after reading a lot of speculation.
After the recent showcase (and even when the first trailer dropped), I dont think it's a mystery at how formations are going to work.
Purely just some of the formation styles:
Ork squads = TWW peasant mob / skaven inf
Terminators (and i suspect tac marines) = TWW Troll
Guard = slightly loose version of more typical formations. Each rank line is offset (left + right) from the one in front or behind with little forward scattering
With that said i'm not saying its a bad or a good thing, that's up to you to decide. I just read a lot of speculation on this topic where i think people may be complicating it in their minds, when the answer is more simple.
Indeed it's early, things can and will change, but i suspect that for formations this is how it is and that it will stay like this.
We'll see though