Not really rare but I'd love to see someone (maybe The Wand Company) have another shot at a TOS tricorder. I wouldn't mind building one myself but it'd be neat to see one done up as accurately as possible, with a working screen and other features too. Maybe even being able to use it as a wireless screen (a bit like a Chromecast or some such) would be really cool.
Something more obscure I'd like to see is a Cobra with a detachable Cricket. For some reason, I thought this existed for the longest time, but after looking thoroughly the other day, I came to the conclusion that I made it up in my head years ago... so now I want to see it made. If (accurate) Cobras and Crickets were easy to get ahold of for a reasonable price (especially the model kits), I'd give it a go myself.
I think the availability of some of these props and prop model kits is a bit lacklustre, unfortunately. Guess I'll have to learn how to 3D model and 3D print eventually so that the lack of model kits isn't as much of an issue for me.