Yeah, it's been discussed a few thousand times, and if you'd like to make it, you're free to try. For me, it would be too much like so many other games, that it wouldn't be Tremulous anymore. I like the asymmetry of the teams. Tribes may not be open source, but I believe they were practically giving it away last year. My point is not whether it would be open source or not, my point is that HvH with bases and buildables has been done, and instead of investing all your time making something which would have a dubious payoff at best, you can just go download that for free and play today. If the open source nature of Tremulous excites you enough to attempt this, I say godspeed. Good luck. I just think you will be spending time doing something that isn't worth doing. That's my personal opinion, and if you don't agree, you're fully free to do as you like.
Cheers!
P.S. I notice that no one ever mentions making a mod where Aliens can play against Aliens. The fascination with all-Human Tremulous seems to me to be the mark of those who are new to the game, and haven't yet overcome the steep learning curve.
P.P.S. I played Tribes about ten years ago, so the idea of Humans vs. Humans with bases and buildables is so last century that it just doesn't excite me at all. Again, to each his own. If you build it, they may play.
P.P.S.S. Would you be so kind as to fix the spelling of the thread subject? I believe you mean: "Both sides human?"