Artificier
A massive, gun-laden fortress on wheels, this has multiple cannons, a turret array, and heavy artillery all on it's sleek, steel-laden body. It can be destroyed by blowing open it's cockpit and destroying the brain inside, then taking control of it.
(NOTE: You could choose to use ACF on this, as it has tank turrets, artillery and other such all rolled into one mod. It is very complex, however, and you may require some learning on how it works.)

Deathmark
A mobile sniper rifle, the Deathmark has never been known to miss it's mark.

Abominatus
A massive hulking robot about half the size/height of Ultima, this electronic monstrosity can be defeated by destroying the brain inside it's head and it's multiple power generators.

The Lost City(SAVE)
A once beautiful, shining metropolis full of life, it's streets are now strewn with corpses and prowled by mechanical monstrosities. A large mechanical brain controls these robots, and destroying it disables the deadly robots.

Satellite Synth
A combine synth designed for orbital strikes, the Satellite Stnth will support the combine, but may go crazy and launch strikes on everything in sight given sufficient damage.

Driller
A medium-large combine vehicle piloted by an Elite, the Driller does it's namesake and drills beneath tbe ground then bursts up, deploying troops and using it's gun-filled mouth to devour and shoot rebels/players/enemies in general.

got even more ideas for you

Moon Dancer
A strange, graceful alien creature, this glides around on the air while firing alien blades at the target.

Lord of Cinder
A large, hulking creature with a flaming sword that is constantly on fire and is in no way related to a certain individual within the Dark souls series

Soul of Cinder
A small, firely little thing spawned by the Lord.

O.R.P.N.(Orbital Rocket Propelled Nuke)
A satellite built by a mysterious race, this has a countxown of about 5 minutes before firing on it's own, though a small control room can, among other things, launch the nuke itself.