Basically an evolution of the training server of old. I think a training server is direly needed for:
Famliarization with maps (so people don't end up playing only atcs and plat23).
Familiarization with advanced evolved alien forms (see this thread, and yes, I've seen complete FPS beginners start with this game).
Giving people something to do when there is nobody to play with.
Core gameplay:
One of the teams is actual players and the other team is bots.
The win and loss conditions are the same as in normal game.
For each room/area of the map there's a list of bot structure layouts, ranked by difficulty.
There's a list of potential starting points and initial bases for the player team.
Bots only rebuild what's already been there, and don't rebuild rooms which have been cleared.
The enemy spawn rate scales with the number of players (in the old training server it was very annoying to play alone due to the number of enemies).
Progress system:
Players start 0 light years into the enemy territory.
Winning brings them 4-6 light years further.
Reconnaissance "missions" start easy (small maps, easy/empty rooms, few enemies).
The further the players are into the enemy territory, the harder the maps get.
Losing sets the players back a few planets.
Losing twice in a row sets the players all the way back to 0 light years.
Players keep their special items and credits and part of the unused BP pool between the maps.
Extra stuff:
Special items, eg. ones which allow you to carry 2 grenades, one-time use abilities (see this thread).
Challenge rooms. When you step on a special pad, it teleports you to a random challenge room where you have to fight off aliens, complete parkour courses, etc. If you finish the challenge, you'll be rewarded with credits or special items.
Maybe a final boss? I have an idea for a final boss, but I'll post it as a different thread since it's so extensive.