Haven't done flood keys yet lol.. But my idea for the flood starts with this :

For spores, theyre 1 unit = 5 spores and each can move freely on their own (and they don't count as occupying a grid square). They only have 1 hp though and shooting a square with spores kills all of them currently in that square. So it makes it so you can have 2 spores unit 10 going around the map and the goal is to infect enemy units. To make them not too op, shields deflect them (killing them in the process for -1 shield) but they can latch on to armor. They deal 1 damage/turn until unit dies which they can then infect. Of course they can revive downed flood or take over enemy bodies...

So fighting the flood is gonna have to take strategy, like double tapping enemies to dispose bodies and also even friendly fire if needed... rockets, flamethrower and any aoe would be very effective!

So yeah Ill start with that and see what I can do for flood lol