There needs to be a good balance of indoor-outdoor areas, with plenty of cover in the former. Part of the current problem is that Spartans, having hitscan weaponry, absolutely dominate outdoors areas, as the bonus of Elite shields means nothing when they can't hit their opponents for issues of sheer range. Their toughness and plasma weaponry gives them an advantage in close-quarters, so there needs to be an equal mix in order to preserve balance without sacrificing team personalities through non-canon weapon loadouts.
On this subject, I'd like to see the Plasma Rifle make a return. Not sure if you're aware, but I think the Repeater is coded to do bullet-style rather than plasma-style damage: it's a lot more effective against health than it is shields. Depleting shields quicker means Elites have more chance of getting melee kills, further boosting their CQC chances and balancing outdoor/indoor gameplay. This of course works better if they're on offence, as otherwise camping advantage would be TOO powerful (think of Shotty Spartans in Boneyard's core room).
The Scorpion has no place in Invasion. It doesn't require a great deal of skill and allows a single man to suppress an entire team, especially considering lack of EFFECTIVE Covy antitank weaponry. A Rocket Hog is more balanced and promotes teamwork, as well as earning a two-man advantage for the Elites for precious seconds after it's been destroyed.
On a purely aesthetic note, of the maps of yours that I've played I was most impressed by those that made use of the natural environment and had Forge objects supplementing them - Jackal Creek immediately springs to mind. I love the balance between natural and artificial here, and I'd like to see maps in a similar style - very much a personal preference rather than a game mechanic, but at least it might save you a bit of budget : )
In terms of spawning, there should be definite safezones with multiple exits to prevent spawn camping or trapping - that's a trick for Slayer, not Invasion, something Bungie really should have thought of when they put in the Elite spawns on Boneyard knowing that the Spartans would have shotguns, the ultimate weapon when you know exactly where an opponent is going to appear.
Also, while the DMR is a definite second-tier loadout, I'm not sure it's a good idea in tier 1 - sure there's more variety, but it's a lot more powerful than the Needle Rifle and with the bigger head hitboxes on Elites really dominates them. The Magnum is a better match as it's less accurate, has significantly greater bloom, and doesn't have the strength of scope. Of course, that doesn't mean the DMR can't appear as a pickup in tier 1, it's just too damn effective at all ranges to be a spawning weapon - the only exception being if the map is set with very short sight-lines and emphasises CQC, where Elites' Plasma Rifles and rush to get in close would balance nicely with the DMR's raw power and headshot potential, but a map like this might have problems providing enough vehicular combat to feel like a true Invasion match.
Lastly, the Banshee is fine. Yes, it forces everyone to keep their heads down and stay indoors, but once they're in it can't really do a whole lot against them. This does prevent the Spartan weaponry from dominating exterior combat, and allows true battles of skill once the Splaser is unveiled - best one wins, loser dies, and that's how its meant to be.
Sorry for the wall of text, I think that covers all my suggestions though. I'm a big fan of your work, and I'm excited to see how any new maps turn out!