I've been putting together a mission type map called "siege". The name based on the gameplay mode that it imitates. (not the gloom map)
The basic premise:
Humans have two bases, no build points, no ckits (adv available), no jetpack, and a lot of pre-made defenses. The aliens have the normal number of build points, no granger (adv available), and a hidden overmind that can not be destroyed. In addition to this, the alien base is filled with poisonous gas that will kill humans.
The first human base should be fairly easy to get past, however there is a forcefield after it, preventing the aliens from reaching all the critical structures at the main base. In order to bring down the forcefield, the aliens have to destroy the repeater powering the base and push a button that is underneath it.
Once this is done, the forcefield is brought down, and the aliens have access to the main base. If they are able to destroy the humans, they win. However, after ~20 minutes a dropship will arrive to rescue the humans. Once the dropship has arrived, a door opens in the main human base, allowing the humans to make a dash for the landing spot. If any human reaches the ship, the humans win.
In addition to this, if humans reach stage 3, they can retaliate. A bsuit can enter the alien base and only take small amounts of damage. Once inside, the bsuit could try to destroy the base, or travel to the back of the base and turn on a generator which will empty out the gas allowing the entire human team to enter the base.
Screenshots:

Drop ship waiting for passengers

Alien base

Generator in Alien base

Dropship flying by the first human base

Inside the human's main base

Door to the escape route

Human's second base

Inside human's second base

Entrance to alien's base