I think it would be interesting to continue following Luke in Ep 9. We could see his spirit traveling to some strange force world (like Mortis) and meeting with other force ghosts (Ewan?) which would shed light on what happens after death, culminating with Luke and the rest helping defeat Kylo or whoever the big bad is. This would be a bold and new direction for the saga and also bring it full circle.
Not gonna lie, I love the idea but sadly I feel strongly that JJ will never even think to stray into this territory. If only RJ could have done episode 9 as well. Which is blasphemy to some people but his bold decisions were great for me in TLJ.