This suggestion is from the streamer Narc; and, it's simple but genius.
Everyone knows that having FOMO cosmetics is not super popular from the consumers point of view. Particularly when the cosmetics are for a game that has not released yet. Intrepid advertises their limited time cosmetics at the beginning of every stream. This does not make a good impression and the newer the viewer, the less good that impression is likely to be.
All you need to do is to change the presentation. The cosmetics are actually content in the world. So, present that content as content rather than cosmetics. Describe the new mount and outfits and other things by talking about how they will exist in game. If the mount is a lizard that lives in the desert, talk about that. Talk about the habitat and how you will encounter the mob in the game. At the end of the section, mention that the creatures and outfits shown are available as cosmetics for the month. Or, however you want to present it.
I think it would also be good to update the front of the webpage as well. Change it from monthly cosmetics to a page that shows some of the new content added that month with the option to purchase that content as cosmetics.
Source:
https://www.youtube.com/watch?v=cUrt1RKT3Kg
While the whole video is about this topic, the most relevant part begins at 10 minutes