While I agree that there surely are modern-day customs and rods that would be great if offered in model kit form, I wonder just how lucrative most of them would be in the long run. In any discussion along these lines, older modelers bring up the kits of "iconic" cars that were produced 40-50 years ago. Yes, it's quite true, more than likely, that some of those kits sold in HUGE numbers, even for the day and age when they were first kitted; but what happened to the tooling after that initial rush of popularity?
Some tooling got altered, modified (fill in your own blank) based on decisions that made good sense commercially in later years (regardless of what we modelers think of those decisions today, what were model companies to do with tooling for kits which had run their course in the hobby shops and variety stores of the late 1960's-early 1970's? Let that tooling just take up storage space, gather dust, even rust?
As a model car builder for now better than 60 years, and having some experience in both the retailing and the kit industry, the car kits I see as having had the longest runs of popularity have been those kits which were primarily factory stock models, albeit with added custom parts, changes of wheels and tires, while still leaving a lot of room for modelers' imaginations and tools to run wild. Of those kits, beyond the very long-in-the-tooth AMT '32, '36, and '40 Fords, I believe the single model car kit having seen the longest production run of all was the original-tool AMT'57 Chevy Bel Air Hardtop, which was cataloged by AMT, Lesney-AMT, and AMT Ertl every year from 1962 to 1996--34 years. Another old kit I see as lighting fires whenever it sees a reissue from that era is the AMT '57 Ford Fairlane 500. One thing I believe we all tend to forget is, that back in the 1960's, the apex of custom cars and still with a lot of hot rods being built and shown, model car kit subjects were a lot fewer in number, and sales were much higher for just about any new release the model companies put out than kits of today.
I can see, almost hear, in my own mind the discussions that would go around the conference table at any company's product planning meeting if the subject of seriously teaming up with a top flight custom car builder for a kit or series of such cars: "OK, so you think this (fill in the blank here) would be a hot subject? Just how many do you estimate we can sell within the first year?" "OK, what can you do as a second release off that tooling, with perhaps a body change?" On and on, such questions do get asked when new model kit subjects come to the table in meetings like that, and they should--tooling $$ don't grow on trees, and wholesalers & retailers will have to be persuaded that any model subject that does make it past the initial planning stages will generate profit dollars for those who will ultimately market the new kit(s) to us modelers. The higher the likelihood of a particular model car subject being a "one hit wonder" with very little possibility of being able to be altered into another version (by adding tooling to the mix, NOT re-cutting the existing tools) the less likely such a subject will make the cut. On the other hand, a model car subject that can, by adding additional, optional tooling say, for a different body style, different engines and so forth will tend to get a lot more positive read from the people who have to approve, and more importantly, commit the capital to making it happen.
That has been my experience, as I am sure just about anyone who has ever worked in the industry at some level.