Forget any game mechanics and view 4E classes just as collection of fighting styles and not as occupations with fluff behind them.
Yup sounds good don't get attatched to the names of things.
Unfortunately I know nothing of this setting
As far as interactions with ghosts, rituals could help out here, making temporary ghosts and the such, magic items which can do the same as well as provide full damage/extra damage versus ghosts.
Don't use 4th edition multiverse if your running the whole thing in this setting just use the 3rd edition multiverse and have feywild shifting replaced by ethereal plane.
Maybe Eladrin having a special connection to this plane and hence with ghosts.
You'd need to make some fluff up for it but I think it could be well worth it, with the Elves splitting and emphasising their connection with living, the Drow maybe wanting to go a step further and use the power of the dead catching them in soul webs (still get a spider and lolth connection here) and extracting their power have the demonweb pits constructed as a massive device to do this. (okay maybe time to stop running with this

)
Have a Ghostouch feat allowing you to see and hear Ghosts with is a pre req for other feats and a class feature/requirement of the special classes/paragon paths
If characters are ghosts consider strongly giving them a insubstantial power similar in effect to eladrin teleport, instead of their normal racial, quite a few racial powers are granted because of the physical form, like Fire resistance, dragonbreath, dwarven minor action healing surges.
Extra Feats could allow the ghosts characters to gain temporary AC once an encounter of a fear like effect
More later...