Cephas,
What I really like is the idea of the rolling squad for the attacking humans. As soon as you commit to attack and not defend, you try to get as far as you can with your health without returning to base (realizing that tight areas can mean dretch headchomp or goon pounce death). When you have to go back to base so as not to feed too much, you slide in, heal, reload, re-equip only if essential (no bumping into people on the medi, hipcheck their camping butts to one side; no playing with loadouts when your buddies are dying, grab a helmet and let's go) and get right back to the squad. So if anybody peels off, somebody else should already be heading back to the attack line, to keep a press on the aliens. A good squad of dedicated humans sticking together can push into an alien base even in S1 (especially during long spawn-queues).
Maybe one human could have something like a ckit, and just a blaster, and as long as he's alive this healing device would have a total amount of health available to the squad. (Yeah, I know, the hated medic option). But just like building, people have to choose to pick those roles. Only by offering the humans "classes" does the game produce teamwork (attack, defend, build*, repair). Why not a healing class?
As it is, I think the thing for attackers to do is communicate with one or two squadmates, choose complementary loadouts, and stick together, or agree to suicide on enemy structures.
*also, by build I mean move and decon with teammates as well as build thoughtfully, skillfully and knowledgeably
P.S. The "medic" would be just like the builder. The healkit would be free, just sell your rifle. Then the humans outside base would have to keep the medic alive, just like the humans back at base better keep a builder alive. Or he'd have to know how to actually use that blaster.
Final note: I don't think that this would be thematically incorrect in the next version, with the aliens using the basi as a medic. The question is, will it even be necessary then?
Also, healkit could be available to work at later stage only, and then near repeaters only (somewhat nerfs/defeats purpose).
Edit: sorry one last thing - to really get the "medic" out of base, maybe have healkit work only beyond the "build line", that is, beyond reactor range (maybe it's battery-powered, and the reactor interferes with the powercells). The "build line" is that magical distance where turrets are not green icons ever, only red. "There is no power here..." (The healkit message would be something like "Interference from a nearby Reactor - or if available, Repeater - is causing the Healkit to Malfunction. Move farther away.")

Ckits and Adv Ckits work inside the "build line" (reactor/repeater range), while Hkits (if you will) would only work outside the "build line". Where you can't build turrets, you would be able to heal people. Either: A) the medic heals you using the healkit (messy for the medic without some binds?); or, B) anybody within X distance of the healkit, outside of reactor/repeater range, presses medkit key, uses healkit health instead, gets 25 health (or something along those lines, maybe a different key, maybe more/less/faster health, maybe just instant health instead of the slow health regen startup, maybe just increased healing rate, etc.). All parameters would have to be play-balanced, of course.
I haven't even thought about the alien counterbalance. I just read your post and thought of this. I'm sure the medic idea has already been beaten back to zombie death already.
PlayerOne used the search feature
and found what he'd missed
http://tremulous.net/phpBB2/viewtopic.php?t=2608&highlight=medicNo increased health for medic.
No special weapon or poison or tesla cannon or gauss gun.
Just a squad-sized pack of health instead of a rifle, some skills with a blaster and the ability to dodge. Make him like the El Presidente from TeamFortress (burn the unbeliever!). No seriously, if squadmates keep him alive, he conveys some benefit to them. If you've got the stones to leave base with nothing but a blaster for armament, your team gets a boost as long as you can stay alive. No? OK.... Thx if u read all of that