So the biggest way to excite the existing playerbase is with a bunch of reworked quests? I agree that going to some of the planes would be cool (Lammania, for example, would make little sense as its very unlikely we could survive there and there isn't really anything there we need to worry ourselves with, though some sort of invasion into the material realm would work) but ultimately I don't think a bunch of upscales to legendary is really going to sell as an expansion to the playerbase at large. I do think the original Shavarath quests could make for a good candidate for legendary, if they wanted to upscale another pack but as they've said previously, upscaling to higher level is more work than simply making a new quest and in general, new quests are more likely to garner the sort of attention and excitement needed to keep a game going long-term.
It is important to understand that there are three aspects; Existing Players, Returning Players, and New Players.
Existing Players will buy any expansion in the same numbers/percentages as the last expansion, warts and all. That's both a blessing and a curse and a topic for another thread.
DDO is old game, what expansion would bring new players in significant numbers?
There are a couple of settings that would attract the most new players; Krynn and Greyhawk. As WotC at this time isn't going to license Greyhawk due to WotC policies, and any reworked older modules and adventures from Greyhawk setting are now set in the Forgotten Realms, we can cross that off the list. Krynn is in active development and publication, however it might be, okay it is, a significant monetary investment by SSG to license I just don't see that happening at the moment.
Myth Drannor and Sigil (Planescape) probably are the biggest movers of the dial for new players, but is it a significant number? Old Game probably not.
DDO has a significant number of players who left the game and drop in to try the game from time to time. We can guesstimate the number of accounts all you want, but it is far more significant than potential new players. How do you get those players back in any significant number.
My thoughts are you update content they liked and you build upon it. They are not going to come back for Myth Drannor or Sigil in the kind of numbers you need. Taking content that is little used (Devils of Shavarath) and putting it in a more logical place so it gets run, adding to the story line and doing the things promised in 2009 would be a good start.
There is also the hints of Dolurrh and Fernia that are scattered throughout the game that would be interesting, but those are more like Adventure Packs.