Not to be crass, but apparently the technology now exists to digitally create deceased actors to interact with real ones.
In this new Star Wars film, in fact, much is being made of the substantial presence of Grand Moff Tarkin. Originally played by Peter Cushing in the original Star Wars film, the role is reprised digitally in this new film, and all reports are that it is seamless. Cushing died in 1994.
So if they need Leia for #9, they'll be able to do it, assuming that her estate is willing to sign off on it.