Here’s a recent example of a similar feature request:
https://forum.inaturalist.org/t/create-a-new-taxon-rank-for-inaturalist-the-species-group/14543
And also note in the curator guidelines some of the considerations that go into creating a new taxon after the rank itself does exist:
While our general tendency is toward fewer nodes for the sake of simplicity, the heuristic we use is utility : if you feel some users on the site (including yourself) will benefit from having a subtribe in the system, perhaps because that’s the lowest taxonomic level at which a particular insect can be identified based on a photograph, then go ahead and add it (provided you properly curate the node as described shortly). If you’re adding a subtribe just because someone somewhere decided they like taxonomic units with less than 20 species in them, don’t add it, because that doesn’t really help anyone on iNat.
[…]
As a rule of thumb, not including additional nodes in iNaturalist is preferable to including but only partially curating additional nodes