Running With Rifles is basically top-down Battlefield. Thanks to a mod, you can turn it into top-down Battlefield: Mass Effect, AKA the game you never knew you wanted. Running With Rifles recently got Steam Workshop support, bringing its mercenary band of mods into the same bunker. People have made some creative-arse stuff for the hundreds-of-people-player war shooter. There's a Napoleonic Wars mod, a race track and, of course, zombies. My favourite, though, is the Mass Effect overhaul, which has been around for months, albeit in various states of completion/polish.
These days, it's replete with an impressive amount of lore-friendly detail. From the mod's description:
-Custom campaign, courtesy of Bredf (RWR forum member)
-3 factions: Systems Alliance, Cerberus, and Geth Heretics.
-49 Weapons; faction specifc and rare.
-40 Soldier models; standard and with armour.
-High ranking soldiers will have better armour, weaponry and AI.
-Each faction can call 4 medics, 8 marines, 5 veterans and 4 minibosses.
Also, if you rank up, you become a key character from the first Mass Effect trilogy, like The Illusive Man, Ashley, or I guess Kaiden (ugh) if I have to.
So basically, it's a total conversion mod that lets you duke it out on large, open battlefields in the Mass Effect universe. You can read about every individual item, unit and vehicle in the mod here. Or you can just download it.
The only real issue is finding people to play it with. These days, there are only a few hundred people playing Running With Rifles at any given moment. They're a dedicated crowd, but not many have the mod installed. People occasionally try to get mod games going on the forums, though, and — worst case scenario — Running With Rifles can fill empty slots with AI.
All told, I think this mod is pretty cool, except for the part where there's not a single Turian unit dedicated entirely to calibrations. If they're not doing the job, WHO IS?