Yeah, I know, plenty of apps have tried to do it. But I tried this one today, and it actually came up with a novel and interesting route!
https://calimoto.com/en
They have an algorithm that, I guess, searches the local Google Maps cache based on curvatures. You can set your destination, or set a number of kilometers in a loop from your current location, and it will auto-generate a curvy, mostly back-roads route there. Then you can do live navigation and tracking along that route, including with offline maps.
The downsides:
1) You get the first map, of your area (the Baltics in my case) free with the app - and others can get pricey. It's something like $5
per week for another area, although if you buy an annual subscription, you get everything for $40. But I suppose it's not bad for a use case where you mostly ride around your home, and spend a bit of money on that rare road trip.
2) It doesn't completely know how to distinguish between sealed back roads, and gravel ones. On my ~200km route today, it included a really awkward little loop in one place, doubling back on itself via an unsealed graded road - not impassable on a road bike, but certainly not fun. Its preference for curves also means it sometimes makes weird navigation choices, like telling me to go down a turnoff from a main road just to turn around and get back on again, or to go around a busy traffic circle instead of an overpass directly across it.
3) Like any navigation app, especially one that you would use with audio and the phone's screen off, it needs to be excluded from your phone's Battery Optimization in settings, or it will just get silently force-closed by the system without you knowing it.
But hey, for what the basic free app gives you, it's a really interesting discovery tool.
