Most of these pertain to how AI allies are handled. As much as I enjoy Firefight, Bungie designed it for co-op, so I believe it needs competent and decently numerous marines/ODSTs to fill in for other players when playing solo. Your maps here are already a bit better about that than vanilla's, but I see some ways you could further improve things without straying too far from vanilla. So, for what it's worth, here's some ideas that may be worth testing:

• Make allies in most cases stay in defensible positions near players' (re)spawn points, and use nearby turrets to better defend against incoming waves.
• Have allies that do get in the line of fire commandeer spare vehicles.
• If possible (and not already implemented), have defending allies take different positions based on their equipped weapons. For example, if you give a marine a Sniper Rifle, they move to higher ground.
• When "Uses Dropship" is set to "Legacy" for a wave, have Flood squads spawn via Flood dispersal pods (from Halo [120_halo]) on every map except Corvette. Maybe also have vehicles during these waves dropped in by Flood-piloted Phantoms.

• Don't have marines stay up near the factory entrance, since a Drop Pod can land right on top of them there.

• Add two more allies and make none of them follow players, since the center of the hangar is a death trap for them. Have them instead hold the elevated platforms on the west side of the hangar, so they can provide covering fire while players move through the center.

• Add a couple more allies that follow players, so it's easier to get troops to man the Warthog without leaving the center structure undefended.

I dunno if these are all very coherent (or that good to begin with), but I hope that it helps.