Simpler approach: Make it a BTB gametype and force one team to be Elites and the other team to be Spartans. Elites spawn with Plasma Rifles & Plasma Grenades and have Covenant vehicles at their base. Spartans spawn with Assault Rifles & Frag Grenades and have UNSC vehicles at their base. Pool of respawns shared by the team. You can gain respawns by completing objectives as they appear on the battlefield e.g. Detonating a bomb, defusing a bomb, capturing a flag, returning a flag, controlling a hill for a set time, killing a VIP etc. The team with the most lives at the end of the timer wins or once the enemy’s lives have run out.
More complex approach: In addition to the above have A.I. backup and enemies(reinforcements via Phantoms for Covenant and via Pelicans for Spartans.), raise the player limit and bring in new larger maps, add loadoat options to customs and perhaps use this for this mode.
The scripting and netcode requirements alone is what pushes this into a hard no territory. Escalation is the only one that might be a maybe, and thats still pushing it.
As for both ideas on invasion here; neither are even close to the 6v6 3 phase capture mode that came from reach, rather both sound more like a modified version of Halo 5’s Warzone with the mostly unwelcome and unnecessary addition of Forced Species just so it can be titled invasion.
If they are to add Invasion as it was in reach to any other game, it should be 4. All of the pieces are there, and for whats missing, its not hard to backport from H2A or harvest forward from reach.
Would be neat to see it in Halo 4 or 2A.
Especially 4 I guess because it never had playable elites, but using a mix of 4, 2A and Reach assets it could.
Not to mention how Elites have cosmetics and armor that would be purely unique to Halo 4. Even Halo 2: Anniversary doesn’t recycle the 4 armor or techsuit, and I kinda love the eczema arms the 4 Elites have, even if their heads are kinda weirdy.