What I mean about "full might" is each faction with its full strengths from its peak to present day 40k. Like both versions of traitor primarchs and it's legions, imperial guard/navy/army and all the condescending versions of the xenos races from its ancient forms to its present state.
NO SUPER WEAPONS. This will be a battle royalist on 100 solar systems outside each universe but having characteristics of each for space travel. Planets cannot be destroyed outright (Indestructible aside from the surface).
Fight!! 😬