Whitetech I realize it is still very early in the development cycle and that is why I am bringing this stuff up. I like stories that dont contradict themselves. A unifying concept of "what the unvanquished universe is" helps those tasked with making the models. The ultimate goal is not as you say to improve tremulous but to create a whole new game. Getting the story straight helps create a unified vibe.
To maintain a logical continuity and to explain the use of trem resources (this is more for storytelling purposes than anything else) a simple solution would be to call it a loosely fitting sequel. If you give the game mechanics a more in depth working over then the old maps and classes no longer fit so well. Defining the goal is necessary step in creating a new game.
The crossbow idea was probably too low tech, same with the melee weapons. Neither will be fun for those in charge of the balance. Anyway the point was to not just reskin the same weapons but to implement a few new ones, even if they serve the same function. It helps alot in differentiating the games.
Buildings
Flame turret
A short range high damage defense structure that excels in tight spaces. Friendly fire possible with area of effect damage. Not something you would put in the main base.
Laser turret
Same function as the MG turret, only with lasers. Highly accurate
Plasma turret
Same guy, different pants.
Alien swatter
This would be along the more low tech lines. A large melee defensive structure that either crushes or cuts aliens that trigger it. Something along the lines of a large hydraulic press, lawnmower blade or very large mouse trap.
Tesla coil
Same as the current tesla but it would look more like the real life thing or the one from red alert. Either a slow repeat/ high damage zap or a low damage multi-target affair. I would like it if it zapped everything within range, alien or human. Humans would be immune to the damage (grounded via their armor) but still get the visual zap. If it is targeting multiple targets you could reduce the damage/ target. Also along the red alert lines multiple tesla coils could charge each other creating a more powerful zap, or was the the allied defense structure that did that?
Flak Cannon
Inaccurate, moderate area damage with variable damage depending on how close to the blast the alien is.
Armory (armor upgrades)
Gun rack (weapons)
Either as two separate buildables or one structure showing the dual nature of the content. Not just a big green box for every newbie to wonder about. If weapon upgrades are tied to various objectives you could have the gun racks as part of the map near the objectives. --> Capture a flag, gain 3 (free) pulse rifles for the team.
Control panel
Gives access to doors and other things around the map. You could lock certain doors and manipulate the world in other ways. --> On larger maps with many different approaches this could mean choosing to exclude part or the map for the duration of the game. Flood the tunnel route while keep the other two routes clear. OR disabling non critical elevators. --> The effect would be to add a new variable to maps to keep things fresh. Cosmetic stuff like lights, locking all doors open
Medical station/ Mess hall
It gives out medkits, no beams of light need to be involved.
Barracks/ Conscription office/ Drop pod/ Surface entrance tunnel
A cosmetically altered telenode
Repeater
Functions like a (very) low power tesla coil as well as giving forward power. It'll help keep the dretches off the forward builders.
Ammo dump/ medical dump
Gives ammo/ medical aid to forward troops without the need for power. Finite amount of heals/ bullets available.