Hello I'm one of those old tremulous players who've played it when it was first released. Years ago, I was waiting for tremulous 1.2 for the longest time and eventually stopped playing the game. After some time I ended playing natural selection 2 because I love asymmetrical type gameplay with fps/rts elements. I've played ns2 since it was released. I saw it become number one on steam. I was also able to witness its decline to about 200 average (there was 1k+ players during its heyday). The devs are currently trying to revive the game. I've seen some of the problems with ns2 that I want to discuss since it will undoubtedly affect unvanquished as well.
On the ns2 forums, there were discussions making it a f2p (free to play) in order to lower the level of entry barrier in order to revive the game. However, this wouldn't have address the player retention (they'd uninstall due to frustration). The biggest problem that I saw in both tremulous and ns2 was the ridiculous learning curve and difference in player skill. To be more concise, getting better at a game the more you play it isn't the main problem. The real problem was the feeling of being utterly useless when you're a rookie. Dying five seconds as soon you leave base and not being able to kill anything or contribute to your team in any way. I'm talking about that pro dretch that can two hit every marine with amazing movement skills during s1 or that pro human that can hold down the aliens with his mass driver that never misses. Similar things also happen in ns2 with players that have over a thousand hours just rookie stomping. It got so bad that they had to implement a skill system and force balance teams before every round starts. Getting destroyed so badly when you're trying a game for the first time isn't very fun in my opinion.
----> In order to address player retention I want to share a new gameplay mode involving bots. I will call them npc to differentiate them from the bots that you currently use in unvanquished. The gameplay mode that I'm suggesting will be a tower defense style gameplay similar to sanctum. These npcs will be slightly slower moving, slightly higher health, and lower attack power. Humans and aliens can build structures where these units will spawn from. The more of these structures they have, the more npc units that will spawn for these team. These npc will spawn in waves with a timer. They can also upgrade these npc units to have more health/speed/damage or maybe to entirely different lifeforms. These npc will slowly march towards the opponents base.
The pro players will have to worry about these npcs in addition to the rookies. Rookies can have fun shooting waves of npc ( kinda like Killing Floor games) without feeling totally useless. Rookies will probably get destroyed later by the pro players but at least they accomplished something for their team. A lot of players start off just wanting to shoot stuff or biting humans and this will cater to that desire. Maybe fulfilling this need for rookies will keep their attention on the game long enough to discover something much more.
Keep in mind that this is just a gameplay mode suggestion... kinda like deathmatch or capture the flag. I'm aware that a lot of old players just want a tremulous 2 so keep in mind that is just one of the many possible game mode.
Also good job so far on unvanquished. It definitely the tremulous 1.2 that I've been waiting for... for almost a decade lol. The visuals are definitely good enough for a multiplayer game. My other suggestion is just optimization/better frame rate <--- this was the other problem that ns2 had during release.... even on high end pc O_O