I’ll go for a whole category. Long range business jets. I didn’t think they’re obscure, yet four years in and not a single long range (4,500+ nm) business jet aside from airliner conversions (BBJ and ACJ).
That may be in part due to the litigious nature of some of the long-range business jet makers. For example, both Dassault and Gulfstream have sent cease and desists to developers if they make representations of their aircraft without entering into a license agreement with the real company. I think Dassault is open to it (and they worked with Feelthere/Wilco years ago on a Falcon), but I know that Gulfstream is particularly opposed to anything that even comes close to looking like a their jets in a non-professional simulator environment.
In Dassault’s case, we have seen some cases of aircraft release (to include their combat aircraft) as long as the dev doesn’t call it Dassault or the aircraft’s real name, while Gulfstream has persisted with takedowns due to saying their aircraft shapes (particularly the windows) are their IP and under copyright protection.
Yeah, I know we’ll never see a Gulfstream, but Dassault Falcons and Bombardier Global series of aircraft are what I expected to see long ago.
I’d love to see a dev tackle any Global or a JS41 which PMDG has done in the past but won’t bring to MSFS or the General Atomics Do228NG.