Alright so what if for next game, we create factions within the HORDE. 3 Colors (RED , BLUE , GREEN). At the start of the game, each color has 2 OZ's. When tagged by that OZ, you join the HORDE but more specifically that color of the HORDE. This would create competition between the colors for largest Zombie group. It would continue that if a red zombie tags a human, that human joins RED.
Instead of having all zombies meet at 1 spot (Hitchcock), have each color meet at their designated spot each time. For example --> RED - North campus BLUE - West Campus GREEN - South Campus. I noticed during the missions that any small objective on south campus was extremely easy. This could make missions longer and more difficult, but of course a better zombie turnout is necessary. Perhaps this could do it!