I thought about ddo2 before, but now I think that to discontinue a game is tragic; look at City of Heroes, how sad
All you need to do is update the old content to run on the new engine and keep it as a secondary starting point; or just make the characters compatible, able to travel from realm to realm, using the old engine, for the heck of it, nostalgia
for your new game you design all new areas, etc. Like use Gygax's world, for example; i had an excellent tabletop map of it back in the day, i forget the name of it now, Greyhawk? I think so.
Personally, I would like to see a true representation of original AD&D, rather than 3.5 rules or whatever. Start from scratch, do a faithful translation to computer medium. Then you can do the same for 2.0, etc. Then each realm becomes an alternative in the same universe characters can travel to. The key is to make characters compatible in some way between realms. If you go to realm 1.0 you follow realm 1.0 rules, so you lose your feat bonuses and gain no feats, perhaps, who knows; at any rate, it is all pie in the sky; ddo will probably die