Great things about the Mercenaries series.
-Large open world.
-Fast paced third person shooter.
-Vehicles! Drive/fly everything!
-Not terribly realistic (the first game felt like as Indiana Jones level of realism).
-Lots and lots of terrain destruction.
Things that Bioware traditionally does great.
-Engrossing narrative.
-Memorable characters.
-Enjoyable and well balanced RPG game mechanics.
-Polished finished product.
Take the best of what the series did, with what Bioware is known for, and it could be a great game. A fast over the top third person shooter. In my opinion, 'Just Cause 2' was a better Mercenaries game than the second Mercenaries game was. Now add in RPG elements and character progression (this does work, look at 'Crackdown'), and some gear upgrading/customization. Keep the airstrikes, but find a way to balance them out. Either make them insane (to recapture the first game's 'shock and awe' appeal) and limit/restrict their use. Or tone them down, but make them far more usable and less powerful (I prefer the former).
Let you create a character (a la 'Saint's Row') and give them an interesting single player campaign. But open up the whole game and story to Co-Op play, and come up with a better system than Merc's 2 that had you tethered to your partner. Put in plenty of interesting supporting characters (including AI controlled NPC partners to help out in lieu of Co-Op), your own base/network to upgrade with matching effect on gameplay (think AC Brotherhood). A number of competing faction would be a plus, each with their own set of iconic equipment and hardware. Take out QTE's for vehicle hijacks, come up with something better.
Now drop me into the middle of this world, and let me have some fun. And maybe, if I'm feeling like it, I just might save the world. You know, when I get around to it. Sorta like Morrowind, Oblivion, and Skyrim...