Contagion Challenge is too easy even on Hard. It's more like irritating than challenging.
Here are a few suggestions, please don't apply all or it'll become NPND.
+ Bloatick Tunnels all around the map, which starts producing Ticks after a while.
+ Cast permanent Malver suppression on the entire map, perhaps by a garrisoned building in the center replacing the Reinforcement Pad.
+ Let clouds deal more damage against vehicles and structures, just like fires in Moltencore.
+ Let Splatters lob BLOATICKS instead lol.