I have a rigid fat bike and have been asking myself the same question. Since I’m selling my 29er hardtail and simplifying my biking experience, I’m going to be using it as my everything mountain bike for a while and I do think that the suspension fork could definitely give a lot of control on the tougher descents. It’s definitely where I’m considering spending my own money besides on bikepacking gear exclusively.
While completely unrelated, the best improvement that I did to my fatbike though was a Jones H bar. With the shorter reach, I can get way back over the rear of the bike without needing to lower a seat and it gives me tons of hand positions so I can stay comfy on the ride to the trailhead.
I rode it rigid as shown all last season, but I will be looking into a fat bike fork. I have my eyes on the Manitou Mastodon.