Here’s my idea for a Flood-dedicated Firefight mode if it’s ever included in a future Halo game. Although it’s unlikely because of the particular mechanics I have in mind for this mode, it’s always nice to dream. (And as a hobbyist game designer, a bit of creative practice is always nice.)
Map and Players
The map(s) would be slightly larger than two of Reach’s Firefight maps combined, maybe about half the size of a Halo 5 Warzone map. Ideally these maps would have a nice variety of structures, including caves, ramps, whatever - it all depends on the map’s theme. For this particular example, I’m imagining a bigger version of the Halo: Reach Firefight map “Beachhead”. Up to 4 players and a few AI marines spawn at a random spot on the map. The AI marines then begin to spread out and patrol the map. Players are given access to a form of REQ station(whether it’s the actual REQ system or a mode/match-specific system is up to you) where players can spawn in vehicles, weapons, and extra AI allies. As with the regular REQ stations, ammo can be retrieved from it for loadout weapons. AI marines can trade weapons with you.
The Match
Instead of having rounds, matches simply have timers that count up; the higher the timer gets basically means the longest you’ve survived. There is technically an objective, however, the objective is completely optional and would vary in “value” based on when you successfully completed it. For example, if you completed the objective instantly it would not be as impressive as if you completed the objective, say, 10 minutes into the match. I’ll explain why in a moment.
Biomass
The Flood would start as nothing more than a small amount of biomass hidden somewhere randomly on the map. This biomass will slowly spread to nearby-locations and the further-spread the biomass is, the bigger the Flood threat; this is where the objective comes in. If players find the small amount of biomass and destroy it(by shooting at it), then the match ends and they’re successful - but it’s obviously not that impressive. If it gets out of control though, then it requires more effort to destroy(since there’s more of it to destroy) and therefore that’s a bigger accomplishment. While it’s impressive to hold out for say, 30 minutes, it’s even more impressive to hold out for 30 minutes AND end the match by successfully eliminating the biomass. Of course, the biomass isn’t the real threat; what is the real threat is the Flood forms that come from it.
The Flood
After, say, 30 seconds into the match, the biomass grows a little bit - and infection forms begin to spawn from it, where they then spread out into the map - infecting AI marines and attacking players. Infected marines may randomly decide to attack players directly, or will drop dead where they are, their bodies slowly converting into biomass where they once stood. With players now being attacked by waves of infection forms and infected marines, containing the biomass(which in this example they never successfully found) becomes a harder task, even though they can trace back the infection forms to find where they’re coming from. As the biomass grows in intensity covering portions of the map, more dangerous Flood threats come from it; Carriers will begin to spawn and carry large amounts of infection forms across the map, Tank-forms(and other pure-forms ideally a wide variety including air-units) will also spawn. The biomass continues to spread the entire time, with the intensity and amount of spawning Flood growing along with it. At a certain point the Flood can also take control of idle or marine-driven vehicles for use against the players. (not necessarily requiring combat forms to drive, but instead using simple biomass in the drivers’ seat like in Halo Wars 2) Also, Flood will at a certain point be able to infect the players’ Spartans, with the player dying and being replaced by an AI-driven infected Spartan complete with energy shields.
In conclusion:
In conclusion this mode features a dynamically and “naturally” growing threat(rather than difficulty ramped-up round-by-round), an optional objective for those who like that kind of thing, a map that dynamically changes in both shape and theme over-time, and a really dynamic and believable Flood experience ripe with mechanics. Since this is just an idea of course, it can be added to and changed however you see fit. Feel free to share some of your ideas for it! Maybe passive animals for the Flood to infect, or player-controlled Flood. There’s a lot of potential here, and I’d love to see it (or some iteration of it) in a future Halo game.