The solution is singleplayer/co-operative mode. I.e. human players vs bots. Note that just the game against bots with standard rules is not what I mean. The standard rules imply that both teams should be in equal conditions. And bots are not match for human players here. So we need something else here.
The following ideas could apply for both Tremulous and Unvanquished.
Tower defense mode
Humans start with reactor that they should protect from the waves of alien hordes. They can build defensive structures like turrets and others.
- The game start in build-phase: there're no enemies yet, players have build tool and some resources to build up their defenses and buy equipment. Players can see who will be in the next wave and adjust their expenses accordingly.
- After some time or when all human players say they are ready, the wave begins. Build tool is replaced with offensive weapon that player selected/bought in the previous phase. Aliens come from their caves and approach to attack the reactor. They ignore other structures until they block the path to the reactor. Aliens also ignore humans until they are too close (that way humans can lure aliens away from reactor). If human dies during the wave the player respawns on some predefined area or pre-built telenode either after the wave ends or after some penalty time passed (can be configured by server).
- After the wave is defeated the build phase starts again giving humans time to recover, refill ammo and build new structures. Then new wave begins and it has more and bigger aliens. And this repeats until humans beat all waves on the level (then they win). If reactor is getting destroyed humans lose. Reactor can't be usually repaired so humans should count every hit on it.
- Tower-defense games usually have the strong separation between the area where defense structures can be built and the path that enemies use to approach the target. The new structure can be introduced: the wall. Humans can stand on the walls but aliens can't jump over the wall or other structures. That way humans can build the maze themselves. Blocking the path should be prohibited or lead to aliens destroying structures on the way to the reactor. If humans build mazes with narrow passages the small aliens will be able to go through the maze, but the big ones may start to destroy structures on their way to reactor effectively freeing the path for smaller ones too.
- The small number of towers. Even if we bring Tesla back it still will be only 3 kinds.
- Limited number of alien bots. I believe the current limit is 64 clients. So e.g. for 4 human players it's only 60 bots. Not so bad actually, but still may be limited.
- Maps. Such mode obviously would require new maps or adjusting the existing ones.
- Mine-dispenser structure that produces some number of explosive mines each wave. Players can put mines where they want to during the build phase or during the wave.
- Classes or special abilities for humans. E.g. slowing nearest aliens, increasing the damage of nearest towers, health regeneration, etc. Player should choose special ability(-es) upon joining the game and can't change them afterwards.
The group of humans start at some area with some low-level weapons. Players should proceed to the next areas fighting aliens and perhaps perform some tasks (collect items, stand against the horde for some amount of time, destroy enemy structure, etc.). Players gradually get new weapons and new equipment and find medicines to recover their health. All kinds of resources (weapons, ammo, medkits and other equipment) are usually limited and should be carefully shared between teammates in order to successfully finish the map. Player don't need money nor build points in this mode.
- Maps. Again such mode would require new maps fitting that mode.
- Connected/disconnected players. It's not clear where new connected players should appear and what are consequences of players disconnected from game. Telenodes can be scattered across the map to allow newly connected players to appear when other players touch that telenode (before that the newly connected players just spectates the game). Same telenodes can be used to resurrect died players (probably with some penalty to the team). Disconnected players can either just vanish or be replaced with bots. Then newly connected player just takes control over this bot.
- Same problem about limited number of clients.
- World-models of weapons and other items since they will be just lie around the level. As temporal solution armories with limited number of free weapons/equipment can be put in some places of the level.
Similar to what Xonotic has now, but with aliens instead of monsters. To make it more interesting we should find a way to enforce players to change their location during the match. E.g. equipment may appear in random places on the map.
Aliens may come in waves too to give players a rest between waves.