In regards to implementation for initial testing, I think the first step would be for me to finish up the game mode system, and have that system allow for adding additional teams and configuring them via attributes, and further add classes/weapons/upgrades/buildables.
After that, I think among the quickest simplest ways to get a third team working in-game with minimal new asset requirements, just to get an idea on how the general idea works out, and the general issues we might have to address after seeing it in action, is to implement RCZ’s robot team. It uses the same human player model, but has a distinguishable skin.
We could then try a team that is based at least in part on the original “droids” team (the predecessor to the alien team we are more familiar with) from when Trem started as a Quake III mod. We could use existing alien models but with new skins, as place holders.
After that we may have a better idea on what to do for a third team, maybe sticking with the “droids” and/or “robots” as at least options, and/or going in a different direction with a completely different team, but in any case after we settle on the overall aspects of this third team, we would need someone to make new player models for this new team. I have virtually no experience in making models for games, however, as I understand it, making organic player models from scratch are among the most involved kind of models to make. We would also need new assets for any buildables (if this team has buildables), weapons, upgrades, etc where applicable.
I am interested in seeing sooner or later a game mode that is a port of the 1.1 zombie mod, and we could try with that tacking the zombies on as a third team (just to see how it plays). Another thing to consider, a third team doesn’t have to be a “proper third team”, for example we could just keep the current human and alien teams, and make it so that the ungibbed corpses of both teams can become zombie versions of the killed class, controlled only by bots, and would attack both living humans and living aliens indiscriminately.