Ok, so I'm pretty sure Grace will make an appearance in Avatar 2, from this article on the Avatar wiki, here's what Sigourney Weaver said:
At the Golden Globes, Sigourney Weaver hinted at the return of her character, Dr. Grace Augustine, in some form, saying, "If you look at the footage, I don't really... I go somewhere, right? It's science fiction. Anything can happen." The actress later confirmed that she and Cameron had agreed to bring Grace back, commenting, "I'm actually in the tree. I can't promise anything, but I think James Cameron wants to bring the family together", referring to Grace, Jake, Norm, and the other humans that stayed on Pandora or helped the Na'vi.
From this, it certainly sounds like she'll come back. But there are so many options....
These are my thoughts on how she might be able to play in Avatar 2:
Jake has flashbacks of when she was alive.
Perhaps when a Na'vi connects to the Tree of Souls, they are able to 'summon' someone to talk to. Maybe Jake does this and talks to Grace.
Eywa really did transfer her mind into her avatar's mind, it just takes a long time for the avatar to respond. (edit) When Grace is dying Moat says "The great mother may choose to save all that she is, in
this body."
This one is a little bit crazy. Someone comes up with a brilliant idea to try to put Grace's soul back into her avatar by connecting the avatar to the tree of souls and pleading with Eywa to put Grace back in her body...... Yah. I know, I know, not likely!

Some of those are unlikely, others not. Anyone else have ideas on how Grace might come back?