For starters, this is a continuation of http://www.halowars.com/forums/thread/109042.aspx. However, I have a policy against drastically editing my topic post with updates for this kind of thread, because those whom asked questions might seem to newcomers to have be answered in the topic post when they really were not at the time.
Moving on, I would like to re-iterate that this thread is in no way suggesting that the Flood should be in single player. It suggesting that the flood be multiplayer only.
We'll start with the basics:
The Flood is a suggested third faction for the skirmish mode of Halo Wars. The premise is that skirmish is not bound by timeline canon, and two factions can become quite boring over time, while three factions can double or triple replay value. Further, the Flood would be a very unique RTS faction of the like we have likely never quite seen before. That's right, the Flood will not be notably comparable to the Zerg. There might be a similarity here or there, but it ends there.
Because of the nature of the Flood, they only have one real structure. This structure is called the Central Intelligence, and has three tiers (yes, I am aware that the Zerg have this too, but like I said, there would be one or two similarities, and none of them are that major). The first tier is the Proto-Gravemind phase. The second tier is the Gravemind phase. And the third tier is the Super Gravemind phase.
Also due to the nature of the Flood, the Flood do not actively seek any sort of mineral. There are no gatherer units and no mining structures. The Central Intelligence will pull minimal amounts of Calcium out of the ground automatically. Calcium is required to upgrade the Central Intelligence and later for Pure Forms. The only other way for the Flood to obtain Calcium is to sacrifice any Flood unit, whether it be an Infection Form or a Juggernaut Form, to the Central Intelligence. Depending on the unit, the player will receive a certain amoung of Calcium.
As more Calcium is pumped into the Central Intelligence, it will grow (this does not affect tier) and the Biomass (warped Flood-infected ground that can be compared to Creep, which is probably the biggest Zerg similarity of the faction) will spread with radial symetry.
From the Central Intelligence, upgrades may be purchased, the phase upgraded, and walls built. Calcium Buildups (walls) are built in a similar fashion to those in Red Alert 2. There is no worker unit. You just click and drag, and the appropriate amount of Calcium will be spent to cause the Biomass to bulge into a wall. Gates are free and can be added by clicking on a section of wall and turning it into a gate. However, one downfall to the Flood gate is that it opens for anyone. However, Calcium Buildups are among the most resilient of walls, because they are tough and will grow back unless completely destroyed.
In the Proto-Gravemind stage, a sqaud of Infection Forms will automatically spawn (one squad of Infection Forms is 7 Infection Forms) on the Biomass every fourteen seconds. Combat Forms are more likely to rush and melee than fire their weapons. Infected vehicles will drive eratically and often try to splatter opponents rather than using the main weapon. Carrier Forms will automatically charge at any enemies in the area.
In the Gravemind stage, two squads of Infection Forms will automatically spawn on the BIomass every fourteen seconds. One Stalker Form (more on Pure Forms later) will automatically spawn on the Biomass every sixty seconds. Combat Forms are just as likely to rush as they are shoot and will advance slowly on enemies while shooting until close enough to lunge. Infected vehicles will have low accuracy, but will take better advantage of the main weapon. Carrier Forms will automatically charge at enemies within a smaller radius.
In the Super Gravemind stage, four squads of Infection Forms will automatically spawn on the BIomass every fourteen seconds. One Stalker Form will automatically spawn on the Biomass every thirty seconds. Combat Forms will fire from stationary positions and only charge when enemies come close. Infected vehicles will act just like normal vehicles. Carrier Forms will have a setting that tells them not to automatically charge, but will still have the option of automatically charging. Also, Combat Forms may be converted into Secondary Intelligences, which are Proto-Graveminds that cannot be upgraded but that spawn Infection Forms (much like infected buildings).
When an Infection Form jumps on an unshielded enemy (or reanimates a corpse), it infects that enemy. If the infected unit was a Human or Elite (or Brute if they end up in the game), a Combat Form is created. If the infected unit was a Grunt, a Carrier Form is created. Otherwise, the unit is simply killed and cannot be infected (Jackals and Hunters).
Infection Forms can also infect vehicles. To do so, the vehicle must be either stopped or moving slowly (or dead, but not destroyed). To infect a vehicle, the rule of thumb is to get the same number of Infection Forms on it as there are crew. Thus, to infect a Warthog or Scorpion, you need two (the Warthog in the game doesn't appear to have a passenger). In fact, most vehicles require only one or two Infection Forms. Scarabs, however, require thirty, who must all successfully climb up the legs. If a transport is killed and had units in it, those that die must also be infected and require additional Infection Forms.
Infection Forms can also infect buildings. To do this, you must get as many Infection Forms into the building as the building has Hit Points divided by two. When a building is infected, it acts as a Proto-Gravemind that cannot be upgraded. Further, if there were any active build queues, the Flood player gets those units, but infected (exception: Jackals and Hunters).
Infection Forms are prone to chain reaction popping. This occurs when one dies and is in a cluster of others, causing a chain reaction of death. There is an upgrade available that causes Infection Form AI to automatically spread so that chain reaction popping doesn't occur as much.
Combat Forms, when infected, will use whatever weapon they had at death. If a Combat Form is killed, it can be reanimated. However, each time it is reanimated, the chance of it coming back as a Carrier Form due to damage to its body becomes greater and greater until it finally must become a Carrier Form. Combat Forms have high resistance to snipers, whose rounds pass right through them. Combat Forms can lunge into the air for air damage (they can also shoot into the air), though it is minimal.
A Carrier Form carries a single squad (that's 7) of Infection Forms. However, it can be upgraded to carry two (that's 14).
Pure Forms will compose a large portion of the Flood's later game strategy. They come in three types: Stalker Forms, Tank Forms, and Turret Forms. The Stalker Form is a fast scout and also kills infantry in a single attack (exception: Spartans and Hunters). Tanks Forms are slow, but have a high melee damage attack (that can also kill infantry easily, though not as a special ability) and high health, and can also spew Infection Forms, either as a mobile spawn point or as an anti-air measure. Turret Forms are stationary, but have a ranged attack that does high damage to aerial vehicles and will curl into defensive balls when under fire for a huge defense bonus. Pure Forms may be morphed into other types of Pure Forms for a small cost of Calcium.
Juggernauts are part of my idea, but exactly how is to be announced (I will edit this in when I've decided). Juggernauts are the strongest of the Flood units, easily able to take on tanks (who do surprisingly little damage, since their rounds pass right through the Juggernaut. Juggernauts may also target aerial units and do very high air damage.
Most Flood units can travel across impassible terrain types. Infection Forms and Stalker Forms can scale cliff faces (and Turret Forms can attach to cliff faces). Combat Forms and Juggernauts can lunge across chasms or up/down cliffs.
When any unit is infected, the Flood player's minimap is updated with all areas that unit has been (however, structures seen on the minimap will only be as recent as when the unit was last there).
If a unit dies on the Biomass, it will slowly be consumed for Calcium. An upgrade is available that will cause Biomass to release deadly spores so that when enemy units stand on it, they will slowly lose health and die.
In the event that a Flood player loses the Central Intelligence, it is not game over. The Central Intelligence will begin rebuilding itself at a place the player selects (the player must have a direct line of sight to build there, and it may not be within a reasonable distance of an enemy base). However, during this stage (called the Feral stage), the player has no control over any of his units. If the player has Secondary Intelligence(s), Flood units within a certain radius of each Secondary Intelligence will be controllable, and the player may choose to rebuild there, which will speed up the rebuilding process. The player can also rebuild at an infected building, though the player does not get the control benefit of Secondary Intelligences from these.
The Flood, like the other factions, has powers that can be used as sort of super weapons. These include Dominate Will, Dispersal Pods, Tentacle Strike, and Omniscience. Dominate Will is a timed area of effect attack that will cause ground-based units to freeze, aerial units to crash, and enemy Flood units to simply change teams. Dispersal Pods fall from the sky and release Combat Forms with randomized weapons over a specified area. Tentacle Strike is a simple area of affect attack that does a certain amount of damage (and is comparable to the UNSC's Orbital Bombardment). Omiscience temporarily lifts the fog of war over a specific area.
When a Flood player fights against another Flood player, the focus will be on infecting ambient life as quickly as possible and then taking Combat Forms back to the Central Intelligence. Infection Forms can reanimate Combat Forms that have died, but they can also take control of enemy Combat Forms before they have died. Once a player moves into the Gravemind stage, things get interesting. When both players are in Gravemind, the game becomes a strategic version of rock paper scissors. The beauty is that it is almost all strategy. Both players have the exact same capabilities and a small pool of units to choose from, so the one that wins is usually going to be the one with the most skill.
Several additions to other factions to help combat the Flood that should be considered are flamethrower units for the UNSC (flame throwers and plasma can be used to target Biomass and destroy it temporarily) and closed helmet systems for the UNSC and Covenant (to protect against spore).