1) A dps/assassin that slips in an out. Whether its incorporeality + power word pain or a borrow speed+x ray vision+death strike, the ability to deliver death without using the standard "walk into next room" technique seemed appealing.
2) A mega-tank. It could be a psion with temporary HP + shared pain or it could be something
a bit more out of the box. The ability to make a mess so that the DPS seems the less accessible damage target seems like a good thing.
3) A support healer. Perhaps its a dread necro and the party all heals from positive energy. Or its a magic-mantled psychic theurge with a relic spamming revivify and pre-emptive heals.
4) A utility radar a la mind-sight + blindsense would work. Or the usual greater invis + truesight + summoning would be good.
If a 5th wheel is needed a diplomancer would be hilarious, as would any cohort-ish build like a crafter with constructs doing the work in a bag of holding.