Wow, some good suggestions while I've been working!
OK with a little more time to think now, (and thinking completely outside the box)
It could do with the backing of a mobile app so that all this nice route planning doesn't boil down to a list of instructions to look out for (unless you google streetview each junction in a printable list, I've done that a few times and it makes life easier than saying look out for the A835 to Small-town-you've-never-heard-of). I know that is VERY wishful thinking but eh.. Go big or go home
Plus if the idea is seeded now and it proves popular then it is the endgame to work towards.
OK so crowd sourcing road ratings, perhaps by 3-4 categories:
Road condition, curviness, scenic, flowing or something like that so that when you plot your route you can choose to have a scenic route or only roads with good condition (some of us HATE surprise gravel patches that make the bike and riders rear twitch)
Ratings might need to "decay" or newer ratings have a slightly higher weighting to keep the information fresh. Incentivise users to submit ratings by achievements or "miles covered" perhaps giving a leaderboard of those who've ridden most this week/month.
The system would be open to abuse but you'd have a report review if it was WAY out of whack with the others.
Social side would obviously be a popular angle but keep it non creepy please, just allow me to see a list/map of roads my friends have ridden and rated in the last period of time.
Groups would also be good for the lurkers among us, like the geographical regions for the forums.
Perhaps the easiest way to implement this to start with is with a google maps layer. Almost everyone likes it for routing so let the layer give you a tracing paper like suggestion of roads between your start and destination with POIs.
How about a collaborative approach too, have the route stored and allow people to edit it (with revisions saved) so a group of riders can refine a route between them?
Export to popular rider sat navs would be good assuming they support complex routing.
As much as we like to think none of us crash, we all do and hotpots can exist, ability to mark down hazards would be good and toggle them on/off so the map isn't clogged with stuff but you can see them if you want. I'm thinking for things like "sheep on road frequently" for moorland areas and stuff along with the "tricky curve here that gets 3 people a month"
Going back to the social thing, how about check ins at POIs so people could see how popular a place is? beneficial or overly social? People could earn... something, some meaningless something in return for posting useful data like a menu for a cafe or something assuming most small stop off places don't have websites. or maybe just a tick list of facilities provided (toilets being a popular request to know!)
Assuming you had a profile on the website you could list your bike or at the very least its range that you get per tank so it could suggest fuel stop locations. (it may sound a bit too hand holdy but it's been done on rideouts and was appreciated by quite a few I think)
are green lanes listed on google? if not allow users to post them so they can search for a route of their own?
Last one, ability to post pictures for the photographically inclined among us, so that scenic overlooks or points of photographic interest can be seen as an option.
hope some of that might be useful but I appreciate it is quite a selfish (and sometimes outlandish) wishlist.