I think the community wants another healing spec, but the demand for a new physical ranged class is even higher, since every new class released has been melee.
I agree with you that the chances of them releasing another melee DPS spec is pretty slim, especially one inside a mech. I do believe that we're more likely to see a Tri-spec class with Tanking being melee, DPS physical ranged, and a healing spec. I really don't think they're going to go two specs with a class that can easily support 3-4 specs and still maintain a class fantasy.
My guess is it would look something like this;
Tanking: Mech spec, similar to Druid bear form with supportive ranged and self-healing abilities.
DPS: Physical ranged with guns, focused on turret gameplay (thanks Rhamses and Thimagryn). Mech is a cooldown similar to old Metamorphosis in MoP/WoD
Healing: Healing beams, Healing turrets, robotic assistants that heal players, etc. Mech form CD to increase healing for brief time. Perhaps the game's first non-mana healing spec?
IMO, if Blizzard goes the mech route, the class will be Goblin/Gnome only. If Blizzard goes more of a GW2 or TL2 style tech class, you could see it spread to other races and tanking could be out of the equation with a possible DPS/DPS/Healing class. I really don't think Blizzard would go pure DPS with a new class.
I like what Thimagryn came up with in regard to turrets and cluster rockets. What about the other abilities like Pocket Factory, Xplodium Charge, Grav-O-Bomb, and Death Lazor?