What? It wont let me put my name in! Dang now i have pen all over my laptop now.
No, not play against the flood, play AS the flood. I think it would be cool if in Halo 3: ODST that you can have the option of playing as the Covenent/Flood/or the ODSTs. And as you progress farther on in the game you unlock other playable characters for each type i mentioned. Like the Covenent you can play as Grunts, Jackles, Brutes, Drones, and Hunters. For the Flood you could play as Human form, Brute form, Elite form, Carrier form, Infection form, and the Pure forms. For the ODSTs you can play as the normal playable characters along with Sargent Johnson, and Master Chief.
However i dont think you should be able to fight the other types at the same time. I only think that you should be able to fight only one at a time.
Ex:
1: ODSTs against Flood
2: Covenent against Flood
3: Covenent against ODSTs