Will the flood ever be playabe?
Probably not, very few people are actually working on this game now compared to what it used to be. The last major DLC we got and will ever get was ATN. Besides, making a new leader would throw the balance of the game into chaos again. Even if it were possible to have the flood in the game I’d still say no due to balancing reasons.
It would make for some seriously fun customs 
Sadly I guess not
Personally, I think Halo Wars 3 should include playable everything. Consider Age of Empires in which there were countless factions.
Playable Factions (Wishlist):
- UNSC (post-2552) - Banished - Insurrectionists (multiple factions - URF, SE, FLP, NCA, SL, etc.) - UNSC (pre-2552) - Covenant Empire - Covenant Remnant (Jul 'Mdama) - Heretics (Sesa 'Refumee) - Swords of Sanghelios - Covenant Separatists (OG Swords - Rtas 'Vadum) - Flood - Sentinels - Prometheans (Didact) - Prometheans (Cortana)
> 2535415876049274;5:
> Personally, I think Halo Wars 3 should include playable everything. Consider Age of Empires in which there were countless factions.
>
> Playable Factions (Wishlist):
> - UNSC (post-2552) - Banished - Insurrectionists (multiple factions - URF, SE, FLP, NCA, SL, etc.) - UNSC (pre-2552) - Covenant Empire - Covenant Remnant (Jul 'Mdama) - Heretics (Sesa 'Refumee) - Swords of Sanghelios - Covenant Separatists (OG Swords - Rtas 'Vadum) - Flood - Sentinels - Prometheans (Didact) - Prometheans (Cortana)
That would have the potential to be awesome and have some very unique factions.