If you need to change the mercs combat role I strongly suggest you turn the merc into a kite class. In order to do this you simply allow Tracer Missile and Power Shot to be instant casts. This will should not effect pve dps and if it does tweak the damage or heat cost of them. But allowing the merc to fire on the run will solve a lot of it's utility problems and not effect it's dps.
I agree with you that Merc dps need some role. Something it is the far and away the best at. Right now everyone objects if Merc gets anything better than what other classes have. Which leads to Merc being subpar at everything.
Dunno if they will make Merc a kite subclass though. The problem here is that the best Merc killing classes are basically unaffected by snares. So in order to have a Merc dps be kite capable, it would need a ton of stuns & roots. In which case it wouldn't need to move at all. Not that BW hasn't already endorsed such a profile (for snipers). But I'd wager that BW would be unwilling to make both snipers and mercs work that way.
There are plenty of options available though if you wanted to make Merc dps viable. Such as:
-Anti-stealth role: give Mercs an ability that makes Stealth Scanner invisible to enemies and roots an enemy stealther that enters its range for 3 seconds.
-Node defender role: let Merc DoT damage stop door/node cap processes (i.e. revert back to the way it worked in 1.1).
-Anti-stun class: give Mercs a talented ability that reduces damage while stunned, rooted or cc'ed by 50%. Combined with the change above, mercs might actually be able to defend a node.
The biggest obstacle to having Merc dps actually have a valued role on a PvP team is Bioware. They need to conceptually get over the idea that Merc dps isn't already the best class in the game. Until BW acknowledges what the entire player base already knows, namely that Merc dps is the WORST subclass in the game, no meaningful changes are going to occur.