I think you could have done something creative like having Wash be the voice of the ship or something like their onboard AI/computer. Something where he is still part of the crew but continuing on with the fact he died in Serenity. I am no writer and I haven't given it much thought until like yesterday when I was hoping for a live action continuation of the story.

Turdinthepunchbowl

This is the form I've been hoping for. They couldn't do a reboot with the original actors in live action and they can't do a continuation with Wash, so they had a tough choice to make.