Heres a few ideas:
For the unlockable character, Im assuming this is one the user can play as? If so, I reccomend the following 2 possibilities.
1. Im thinking a camel based off an old Staghound chasis would be pretty cool. Its similar to a tank, but has 4 big wheels instead of treads. The neck would stick out elongated and it would spit when it fires.
Also, to make it interesting, you can have a missle launching system built into its hump to defend against air attacks. The hump could suck in and pop outwards for the attack animation.
(I will include a few pics shortly)
2. Theorhetically, helicoptors werent actually used as weapons until the vietnam era, but hey, this is a game, you can do whatever you want to do right?
Anyways, my second idea revolves around of all things, an elephant. He would have helicoptor blades attached to his back, and his little tail would spin as well, acting as the stabilizer.
His first attack will be his trunk that shoots out explosive peanuts in a machine gun fashion.
His second attack will of course have to be flaming turds that drop to the ground igniting whatever it touches.
I had a idea for a boss character that would be a tough customer. Im thinking of a Rhino with extra armor plating that has above average speed and a horn that rips thru his foes.
Also, the horn could shoot bullets or grenades, as well as having a possible turret on his back that has 360 degree motion and fires off random volleys of sticky bombs that slow your movement.
I will try to add some pics of what im talking about in a few.
The game looks like its going to kick ass, keep up the good work.