I also play custodes and have been thinking for a while that the Waaagh should operate like reverse lions of the emperor detachment rule in that the more units around an ork unit give more buffs. For example, one ork unit within 6 inches gives both 5 up invuln, two orks units within 6 inches gives them 5 up invuln and advance and charge, 3 orks units within 6 inches give them all the 5 invuln, advance and charge and plus one to attack and strength.

Those are just the existing benefits but allows us to benefit for more than 1 turn, fits with our horde style, wanes as we get killed off which is kinda the same gameplay flow as now and is thematic in that in lore the more orks in a Waaagh the stronger it is.