I used to play Mercs4 pretty frequently. In fact I was on the beta team for the first two Mektek Mekpaks (look for Suralin in the credits for the first one; about halfway through the second one my compy exploded and I couldn't play for a while).
These days, tho, it doesn't appeal to me as much as it used to. I do play Megamek on occasion (also under my Suralin alias) but I'm only intermittently active.
I have a thing on my HD (and on my USB key) that has detailed info for how I would craft a MW5 -- the plot, mission by mission (Grumium: Jihad vs. FRR-and-you vs. Wolf); the slottage for various mechs; the way multiplayer would work (Good luck trying to boat in league play, when your unit doesn't have infinite weapons and can't repair instantly...); how fighting in melee would work...
...I have this image in my head of an Atlas grabbing the corpse of an Urbanmech and beating a Madcat senseless with it. God, that would be awesome...
But alas, no one has plans to make a Mech5 at the moment...