I don't know what time of year this could be used for...possibly summer or just select weekend night like Mad T Party, but I think a nightime event for GP would be pretty cool. The event would have a couple of small experiences you could do as well as an Improved Grizzly River Run.
Glow worm "light show"
Glow worms (represented by small lights) would be carefully hidden throuought Grizzly Peak. At night, these lights would slowly begin to glow brighter and brighter until they light up all of grizzly peak. They would remain on for the remainder of the event similar to the neon lighting in Cars Land.
Puppets/ Live animals
Larger animals like deer, Raccoons, wolves etc would be represented by puppets or small animatronics similar to Olaf over at the frozen meet and greet. Smaller animals (certain insects or reptiles etc) could be on display similar to the pony express shack at Knotts Berry Farm
In RCCC kids can join Russel, Carl, and Doug where they tell campfire stories and songs
Native American activities
Guest can participate in Mono and Sierra Miwok games and crafts.
Grizzly River Run additions
Every 30 minutes (hour maybe?) there would be a small light show telling the legend of how grizzly peak was created. For those of you not familiar
“ Long ago, Ah-Ha-Ee, the Coyote, met Oo-Soo-Ma-Te, the Grizzly Bear on top of the mountain. Seeing the Grizzly was a powerful being, Coyote asked him to watch over and protect the land. Then one day, people came and tried to chase Oo-Soo-Ma-Te from the Mountain. But Grizzly was strong and held his ground. When coyote saw the brave bear standing alone against so many, he turned Oo-Soo-Ma-Te into stone so he could never be driven away. To this day, people claim they can hear the great bear spirit in the wind that roars through the caverns and trees of Grizzly Peak.”
―The Legend of Grizzly Peak
The light show would end with the mountain coming to life as a grizzly bear before turning back to stone using projection mapping. On the ride, Guest may be able to sight Ah-Ha-Ee the coyote prowling the area watching the grizzly bear watching over the land.
Bigfoot takes over the Grizzly peak trail.
While walking through Grizzly trail, guests may be able to hear or even spot the Sasquatch that is rumored to roam the area.
That's all I have so far, if anyone has any ideas feel free to post them below. Maybe the event could spread to other animal focused areas of the park like A Bugs Land or maybe Pacific Wharf.