Could someone talk you into creating a way to add in their own vehicles?
It could provide a lot of creative liberty to your userbase and could easily be superior to other vehicle mod's systems simply because your physics are smoother and everything is more streamlined.

Because .json driven resource packs might require a rebuild, perhaps instead you could add an API and have some extended conversations with someone (like myself) willing to help write the docs necessary to teach folks how best to handle implementing all their resources?

Perhaps it's just too much work, but you've built something great; perhaps inviting everyone to come out and help make it bigger is the next big step. (Plus, hosting a vehicle-sharing forum on your site would generate more traffic and you could add "best of the community" videos where you test out folks' creations.) It'd continue to strengthen your viewership.