I agree. Custom routes are already sort-of possible just using MyMaps for Google Maps but it’s a bit crap and unwieldy. It’s probably something that’s better added to Metr Pro (@rpasichnyk) as a groovy new ‘publish’ feature though, rather than something to be completely reinvented.
The most worthy software challenge I can think of is the mythical and highly sought after Mellow-esque ‘Endless Mode’ that gives you power-assisted kick-push for stealth boarding. There’s already quite a few threads on this subject cos many lesser folk have tried and have failed.