I would love if Anakin made a return in this manner. We saw how influencial Obi-Wan was as a force ghost, and then, in ROTS, they teased about how Obi-Wan would be able to contact Qui Gon via the Forca (aka Force Ghost). But they didn't show it. Obviously, a force ghost is pretty important if that was essentially how Luke finished his training and was something Yoda brought up to Obi Wan when they were the 2 remaining Jedi. You don't introduce a concept like that, then never use it again (granted, it's Abrams doing these now, and not Lucas).
Honestly, I can see it happening as Kylo goes however far into the dark side that he'll eventually be at. He wants to "complete what you (Vader) started" and then Anakin comes in and is like "yo, I was wrong, grandson"
