Your focus should be on playability not the class mechanics.
What I mean by this is that the merc has the worst ability lineup for maintaining separation with their opponent. The sniper seems the best at this, with knockback root, stun, mez, multiple defensive abilities. And their cover makes it impossible to jump to them. The sorc has knockback, stun, mez, snare, root, shield, speedburst. They are decent at kiting. But a merc has stun, mez, knockback and def ability. They got the short end of the stick. I would say that what you need is a snare on one of your primary abilities so slow the approach of an enemy, and maybe get something like a jump to friendly target or aoe snare to kite with. As it is its not your damage that hurts you, but the fact that I as a melee have no problem keeping you in range to beat on till your dead. Maintaining separation should be your focus.
Arsenal has a 70% slow on Unload for the exact purpose of slowing melee opponents approach.
You CANNOT maintain separation with melee when you are as dependent upon casted abilities as Merc is. The best you can hope for is 1-2 casts before the melee is on you again, and that is assuming they do not have a leap cd up. Maintaining separation is only pat of the problem.
I will gladly accept the fact that melee opponents can catch up to me, if I am still allowed to maintain my DPS while running. If you stand still, you are dead against a melee opponent (unless they are terrible). And in the end, this is a team based game. There are plenty of classes that ooze utility in the form of roots/snares/CC, let them keep that role and allow Merc to do its role effectively
And my focus IS on playability..... every change I have suggested would improve the playability of Merc.