I’ve found that it is worthwhile paying attention to plant pests – galls, leaf mines, mildew/rust, etc. You may have to research them yourself if you want an ID, but since they are generally host specific, it is a good way to record species that are often not observed very often on iNat. There are a number of forum threads and iNat projects on galls that may be helpful.
(I was going to suggest soil fauna, but after reading your profile I assume you’ve already covered this.)
You also might look at this thread for inspiration: https://forum.inaturalist.org/t/fungi-on-seeds-and-fallen-leaves/37136
Or, possibly, this one, on organisms in novel/artificial habitats: https://forum.inaturalist.org/t/stealing-candy-from-barrel-fish-the-sequel/36822