In any case, a "floating" personality seems very likely, for a lot of options from "saint" to "spirit", as would a ghost polymorph someday... after all, we did get several flavors of skeleton, draugr and ankh-ra as I recall...
Lich outfit pieces exist in the game, and could easily be given to players in some way (crown store, or event reward like the companion visuals...), together with a soul-shriven skin and a floating personality that would make for a decent "lich!" setup, right?
Nereid... well, that might also come someday. Either as polymorph, or as outfit & skin combination... might be nifty if they ever make a "high seas" themed expansion...
The only real question is when they might get to a good point where they might think about doing those.