While I've heard that the Druid class is going to be appearing in DDO (which would round out the 'core' class list), there is one character class that is totally unique to the Eberron setting, and is actually essential for what the setting itself entails: the Artificer.
Simply put, the Artificer embodies the reversal of Clarke's Law, thusly: "Any sufficiently advanced magic is indistinguishable from technology." Artificers were responsible for the creation of airships, elemental galleons, the lightning rail, and the warforged. They created many of the magical innovations that people in Eberron take for granted -- perpetual lighting in cities, cargo-moving cranes in harbors, House Sivis's speaking stones. Most of the magical gear that adventurers rely on were made by Artificers.
In DDO, they would be the middle-ground between the different types of spellcasters. They have good armor and weapon selection. Their spells run the gamut of healing spells (they would have both Cure X Wounds spells and Repair X Damage), along with buffs (especially those that could be applied to a character's gear). While not a direct-damage type of caster, they would be alongside bards for a support role.
Not to mention their mechanical bent gives them access to the skills Disable Device, Open Lock, and Use Magic Device. No longer would rogues be the only class to "trapmonkey" (much as I hate the term, it fits here).
Possible enhancements for the class would be anything item-related. They should be the undisputed masters of getting the most oomph out of a wand, or maximizing a charged item. Perhaps they could even be given the ability to restore lost charges to an item, getting a little more use out of something before it runs out altogether. One could also argue in favor of giving Artificers the ability to repair damaged equipment, maybe even restoring an item that has broken in the middle of a quest.
Within the class-grouping, I'd have to say that the Artificer would probably fall best in the "Specialist" category, alongside rogues and rangers.
So, who's with me? Who wants to see the penultimate tinkerer make an appearance in Stormreach?