this is already possible. you have to initiate such a subscription as a place subscription, and then you’ll be able select a taxon to go with that place.
this may be best discussed here: https://forum.inaturalist.org/t/notifications-on-inaturalist-please-share-your-ideas/374. i made a suggestion there that i think essentially gets to the same result as what you’re suggesting here: https://forum.inaturalist.org/t/notifications-on-inaturalist-please-share-your-ideas/374/37.