Just so you know, I'm still working on this. It just got sidetracked for the last few days due to holiday stuff. (Also,
Kerbal Space Program.) I'll have some work-related things coming up for the next couple of weeks, but I have a lot of free time in that period, so there WILL be a new mod posted soon.
No. I mean, yes, I considered it, but doing that would really screw up a few other things. Realize, I have six custom Combat Classes for units: Myth, Psi, Energy, Multirole, Orbital, and Titan. One in the myth mod, and the other five in Ascension; the five later ones weren't created because I just wanted to make new classes, they were created because the existing classes didn't have the characteristics I needed, or had other limitations.
Some unit types (and the Trance promotions) give a bonus against Psi units. Changing Psi units to be some other unit type, and solely using a promotion to denote their "Psi" status would break this. The same goes for Myth units; the entire Hero mechanism would break if this happened... unless we gain the ability to make a promotion that gives a bonus against another unit with a specified promotion. (I'm sure we could do this with the DLL.) This is why Hero units ARE normal units with a Hero promotion stacked on; I wanted them to be vulnerable to all of the usual anti-whatever promotions. For Titans, the problem is the reverse: I made them a new combat class type in part because I wanted them to NOT be susceptible to any anti-X promotions; you can make anti-armor units, or anti-infantry units, but nothing gets a bonus against a Titan.
Then there are the promotion issues. Promotions are made available to units based on their combat class, so if Fighter units can't pick ground-attack promotions, and Bomber units can't pick anti-air promotions, then what do I do with my Multirole units, which have access to both sets? (And remember that Jet Fighters and Stealth Bombers both upgrade to Needlejets, so making those be only one type would lead to other problems.) Titans and Myths needed both ranged and melee options, although that's a lot less problematic since I merged the two lines. And then there are Orbitals, which are air units but whose promotion list is a lot closer to the Siege set; if I made them Siege units outright, they'd have access to certain promotions that make no sense for an air unit.
Pretty much the only combat class that wouldn't have issues is Energy, since they're basically just an updating of Gunpowder units. While I wanted them to not be susceptible to anti-Gun units, and to be eligible for a few promotions previously limited to armor or helicopter units, I can compensate for that in other ways.
This will be a big part of my upcoming design changes. That is, once I post the "functional" version of the mod, I'm going back and work on doing it "right". A big part of this redesign is breaking up some previous promotions into multiple parts. For instance, the Psi promotion (which all members of the Psi combat class have) gives a bunch of abilities; besides the ability to adjust strength dynamically, they also have x2 XP gain but no great general points, can move through borders, and so on. Now that the promotion cap is gone, I can split these into multiple promotions and make the Psi units a bit more distinct.
Of course, giving every unit more promotions means more icons on the screen, but I'd already updated the unit panel UI to be able to display ~20 promotions instead of the usual 11, so it should still be fine.