I would have no problem with it as long as they take it seriously. Ewan is great, he'd nail it. I'd be ok with Christianson as long as they did some voice coaching and make-up to get him looking and sounding more like Shaw at the end of Jedi.
On the other hand, Luke is old and wise by now and shouldn't really need guidance, so his force ghost buddies could be off playing shuffleboard in Jedi Afterlife too.