Setting:
Mars, mainly wasteland deserts and some fringe city locations. Future, where Mars is colonized. Only one ‘country’ currently on it, formerly run by a generally competent government that was overthrown by a fascist Empire that is efficient, ruthless and only really in control by the use of fear and force. Those still in favor of the previous rulers are either driven out of the country violently and hunted down until killed or rounded up and executed. Main fighting vehicle and force consists of mechanized walkers, ranging in size and purpose to be anything from scouts to brawlers to mech hunters similar to tanks of the modern era. Infantry are still around, but most of the work is done by these mechs, and most weapons are designed to fight them as well.
Plot: Main characters would be part of these rebels, with the opening scene being that of them fleeing, on the run for some time with most of their mechs damaged and either breaking down or completely non functional. As the rp drives on, the main focus of these rebels is to take back their country, and essentially save the world.
Technology: Mechs, obviously, sophisticated enough to have jump jets on some and be piloted by only one person. No energy weapons exist at the start of the rp and are in experimental phases somewhere in the middle. The rebels would have a much lower level of technology than the Empire, having to scavenge and improvise often, sometimes in mid battle. Weapons would mainly be things like cannons and machine guns, scattercannons and rockets and the like, with melee weapons being primarily used by the mech hunter classes where available and designed to be used as such. The mechs are not fast, at least not fast by mech standards, but are faster than some tanks today. Their durability is determined by their class and design, just remember that the rebels would have older mechs, and would possibly have to scavenge from older wrecks as well. Infantry have things like grappling guns and magnetic clamps to help them combat mechs, along with jetpacks. Some jetpacks allow for a full on aerial fight, though those are generally Empire only.
There's more to be had, but this is the basic idea, the details won't change any of this, just the fluff of everything.
Also blame Kristy for yelling at me until I put this up. This was originally just a joke idea.