My opinion:
Is there interest? Yes, people will always say they are interested.......but.....
Is there a willingness to pay a $40-$50 price point?? I'd say possibly, with the qualifier that it depends upon quality and the "value for the dollar" present in the kit--but probably not in numbers that would make the project viable over the long term.
There's a lot of adjutant accessories items that are desired for G.I.Joe/COBRA that have never been made in scale. The COBRA throne is one of them. Battlefield diorama items are often asked for but its the SPECIFIC items that no-one can seem to agree on?
Bunkers? Emplacements? Observation posts? Lots of possibilities.
The right one will reach out to a lot of imaginations and fuel interest and demand. The one that might be a great set on its own, but that doesn't grab people.....can kill the whole exercise in a single blow.
Putting effort into something that doesn't sell.......even if people ASK for it......just murders any continuing incentive.
So, therein lies the risk.
The other thing is that when it comes to putting money where the mouth is.......a LOT of collectors are fickle AND cheap. A $50 resin kit.....even an exceptional one with well-fitting parts, and great detail.....is a tough sell. A lot of people cry for such things from the rooftops, but when the sets are made and its time to step up and order with real cash money.........you hear excuses like......." Oh, I can't paint" or " I don't know how to build resin kits" etc. That or " this kit only has 6 parts and you want $40 for it?

".
Its heart-breaking to the well-intentioned creator.
But all that said.......
I think the adventurous creator, who is willing to take a risk, and can produce quality, detailed items........has an unlimited playing field in front of them.
Even if all you do is think small kits.......
There's the aforementioned COBRA Throne
-a Brain-wave Scanner
-new cupola for the original/25th HISS tank, with modified weapons.
-new weapons for the new POC HISS tank, or some of the other vehicles with the new peg-hole system.
-small items like gear to be stowed on the new VAMP, like bedrolls, jerry cans, ammo crates. etc.
-a "war" table for the Joes--a status/map board like computerized table top console, like would be found in the PITT.
Maybe items to take a G.I.Joe vehicle and turn it into a diorama...........such as a display base with a partially buried fuel bladder, so the display can be about replenishment. Maybe even just a section of ruined wall, with printed posters for a COBRA Embassy ( or a US one for that matter). Something simple that can be the launching point for those with skills, or can be a relatively quick and simple display for anyone that can glue two parts together and wield a paint brush.
Obviously, size means more casting materials and means more cost...but lots of ideas are possible.