DK said they haven't decided what to do with Revan... so what happens from here... who knows?
Clearly at the end of The Foundry FP Revan can see he has been driven close to madness with his genocidal plan and before the end he quotes Malak before... ? nobody really knows what happens next with 100% certainty.
Is he a force ghost... that would be fine with me opens up some nice Knight Story-lines /w Scourge.
Did he use Fold Space and is now somewhere else recovering... that would be fine as well, I would use this to make Revan completely morally grey and also use it as an opportunity to reintroduce another KotOR character, Jolee Bindo.
Keep Revan hidden away and let Jolee show him the path of the "Grey" Jedi, then some time down the line I would have Revan return and do something decently heroic before moving on... maybe a little force ghost Bastilla before Revan dies.
At some point BioWare will release an expansion dealing with a threat both Republic & Empire will need to fight off having Revan return and helping both sides here before his death would be a decent solution.
Don't make him a boss fight again... give him a fitting ending or just let him reappear as a force ghost.
*Edit*
Just to clarify I would like the Revan in Exile story to be a book not in-game... okay get on it BioWare.