A couple of fun ideas for the class that would never happen, but add to their diversity a bit:
First of all, the new class should use a new resource, which is pressure. Certain abilities should drain pressure, while others should pressurize the suit (like rage, i guess, but without the decay.) Tanks could even divert damage into "Steam Leaks," causing no lessened damage to the player, but causing a leak in the mech suit which makes generating resources more difficult for a short amount of time. DPS could have a lootship like mechanic where they get over-pressurized at 100% steam buildup (which gives DPS a ramp up time, and a potential mastery) causing less DPS until pressure is reduced under 50%. Healers, of course would have a passive talent (Reserve Tank?) which would allow them to heal without worry of the resource mechanic.
New armor class: Mech Armor
Every tinker is provided three suits of mech armor when they graduate tinker academy (starting zone). Each mech suit can absorb a different set of gear. Mech pieces function as heirlooms in that their armor scales with level, at approximately 80% the armor value of plate. Mech armor inherently has no stats.
Spell: Mech Armor I, II, and III
Scans a piece of armor into the currently equipped mech-suit, causing the suit to absorb the properties and appearance of that armor. This process has a small (0.01%) chance to find a pattern and produce a superior result from the original, or to cause a faulty scan, creating a slightly depreciated copy of the armor. The armor is consumed in the process. 1 hour cooldown. Not usable in combat.
(This allows the tinker to "equip" any gear that drops. I think the tank spec should be switched to strength.)
Backpacks (runeforging-style enchants, for cloaks.)
The tinker's vast arsenal of research, medical, and offensive supplies requires he keep a pack handy at all times. Different packs can be chosen for different situations, and each pack has a unique visual style.
-Medkit: Chance on successful spellcast to increase your spirit by 10%, and to also increase your target's primary stat by 5% for 15 seconds. This effect can only occur once every 30 seconds.
-Extended Mag: Allows the tinker to withstand 30 more PSI within the mech suit, and reduces the amount of DPS lost whilst overpressurized.
-Iron Warbag: Contains the technology used to fabricate Jeeves, allowing passive healing over time to the tinker's robotic arms.