Given that we have no real information, and given that these writers can do what they want, including introducing new Force abilities, here's a thought...
In TLJ, we learned that somehow, even though he's dead, Yoda can actually use the Force to do things other than talk to a Force user. He was actually able to cause lightning to hit a temple and destroy it.
Of course you have the obvious question of "why doesn't Yoda just bring down the First Order?" He can't be harmed because he's dead, and he can really screw with people, but let's ignore that.
The point is that like it or not, the ability of a powerful Force user to affect the physical world after he dies exists. That was first introduced in TLJ, and by the same logic, it's also possible to introduce other things Force ghosts can do.
So picture this thought--Luke's death was quite stupid in my opinion. I feel it completely ruined TLJ for me, more so than any of the other stuff for many reasons.
But Luke was using the mental part of the Force, not the physical. When it was done, he simply disappeared into the Force, even his mechanical hand.
What if this is a previously unknown Force ability? What if Luke willed himself into the Force, and by the same token, can actually will himself back?
As long as we are speculating, I think such a move by Abrams might be a good way to win back some of the alienated fan base, especially if once Luke comes back, he does what people wanted him to do in the first place.
