A little bit more DPS potential would be nice so we could be more competitive with Snipers and Maras. I think that if heat was a little bit more manageable that would be nice. I know we can technically get by, but we can't really deviate from our rotation at all if we want to keep heat down. I think that making Unload proc more frequently, at least for arsenal, could help with that and it could increase our DPS as well. Or you could have rapid shots actually lower your heat instead of just costing no heat.
I think heat and concussion missiles cast time are 2 issues. Merc already parses close to a sniper on a dummy but does even better in pvp because it has 55% ignore armor on all its attacks and 85% on unload and railshot. Also merc has healing that sniper doesn't have(kolto overloads,bomb,etc) and more mobility.