Thanks, tiwane!
I always thought it would be cool if iNat did a sort of “community annotation challenge” to help annotate the many unannotated observations for a particular species. Using the Monarchs example again, there are close to 30,000 unannotated, research-grade observations according to the “Identify’” settings you mentioned above. 
A few other relatively large citizen science initiatives I have been a part of hosted similar challenges multiple times a year to really focus a majority of their community’s power to completing the same project together. For example, the “Monarch Community Challenge” could be advertised on iNat’s home page for maybe a week or two before the challenge begins to educate unfamiliar users about the Monarch, and teach them how to properly identify and annotate an adult, pupa, larva, or egg. Since these stages aren’t very hard to distinguish between, I think it would be pretty easy for almost anyone to learn the 4 major life stages and even sexual dimorphisms for the Monarch.
The challenge could last for a few days, and those participating in this initiative would work to annotate those 30,000 posts for however long they please. I’m not sure if there’s a way to keep count for annotated observations, but there could even be a “Top Annotators Leaderboard” specific to the challenge, without “game-ifying” the process too much.
As you stated, really cool data could be generated from having such a large number of annotated posts, especially for popular species like the Monarch. These challenges could be useful in the future with other species. For example, if the Spotted Lanternfly continues to spread over the coming years, I’m sure it will become an even hotter topic for scientific research, and iNat could supply more information through annotation metadat. Another positive would be getting more eyes on research grade observations to catch potential mistakes or mis-identifications, and I think it would build an even greater sense of community.
Some potential downsides could be an influx of users annotating a species that they’re relatively unfamiliar with. It may be easy to annotate for the monarch, but other species’ stages may be harder to learn and distinguish between.
Sorry if this has already been discussed somewhere in the forums. I searched but couldn’t find anything. Thanks again!