This is a theory that I've had for awhile. So, we all know that when a StarClan cat or a spirit gets killed it never comes back, right? Examples of this are Spottedleaf, Bristlefrost, and that Dark Forest warrior that died in battle with Ivypool. Just a recap, Spottedleaf died when she stopped Mapleshade from killing Sandstorm, Bristlefrost's spirit died in the Dark Forest when she pushed Ashfur into the dark water, falling in as well, and we all know how that Dark Forest warrior died.
I think that once their spirit is dead they don't actually disappear forever. I believe that once they die or fade that they become invisible and unable to be heard, felt, or smelled by other spirits except the other deceased spirits. I don't have much evidence for this theory, but it would give us a nice happy ending to know that Bristlefrost isn't actually gone forever and because she's now a deceased spirit she can't be detected by anyone else. She would also be in the Dark Forest, trying to escape which gives rise to a new theory.
What if when a spirit dies it actually goes to the Dark Forest? This one is also far fetched, but to me it makes sense. I also think this would be a good explanation to why the StarClan cats are probably afraid of being killed instead of fading. This gives rise to a whole new thing of evil protagonists. Multiple times I have found myself imagining an evil Spottedleaf or Bristlefrost (or both) working with a villain like Mapleshade. Like I said, there is little evidence for this, but it's still a very interesting theory.
Another theory I have is that cats from StarClan reincarnate more often then we realize. This is actually sensible. I often find it convenient when a cat who has the same pelt color and prefix as a cat before them appears. It becomes even more convenient when they have similar attitudes. This gives rise to my Graystripe is the reincarnation of Gray Wing theory. Think about it for awhile. They have extremely similar attitudes and pelt colors.
Tell me what you think and what your theory's are in the comments. I plan on making theory book soon to put all my ideas in. I'll do more research next time on my theory's and do yours to.