This is what I want to do - and I think many people probably would too:
Plan a route by studying a map - preferably OSM of course
Plot that route on the map
Export it as a GPX file to upload to your GPS app
As far as I can guess, this ought to be a very simple and straightforward thing to do - yet, it appears that it isn’t possible. What I have been able to find is a lot of ‘route planner’ applications, all of which decide for you which way to walk, ride or drive, and which are at best very awkward, when you want to design a track entirely on your own whimsy. I mean, I might well want to go completely off-track, or choose a wildly inefficient route to get from A to B; route planners don’t help you do that.
So, with the rant out of the way, have I missed something? Is there, somewhere, this basic functionality, say in JOSM or Merkaartor, to simply plot the track point by point, perhaps even just as straight lines from point to point disregarding paths and other topological features?
I prefer http://www.gpsies.com/createTrack.do
After signing up and logging in for free you can make you own tracks using osm or google following roads or direct for sections where there is no road mapped and save and download as gpx. You can change the background maps as desired whilst making the track. Using full screen is best. Then you can copy that to where your gps app likes to keep and access those.
Anything like this would be a third party development, as it doesn’t contribute to the contents or accuracy of the map.
My guess is that, as a small subset of an automatic routing application, it is not challenging enough for developers. If they wanted the function, they could probably knock it up with low levels tools and simple programming, but this would result in something that was too techie for the main in the street.
The other thing that might get it done is if someone could come up with a business model for the service, probably based on advertising. There is a risk that it would be too easy, so difficult to create a unique selling point for the site, meaning that there were be lots of competitors, all only getting a small part of the market.
If I wanted to do it for my own use, I would probably map the route as a relation, in JOSM, export the relation, then use some simple scripting to reformat the OSM file into a GPX. It is possible that this last stage is available, ready done, somewhere, or that later versions of JOSM allow relations to be output as GPX (presumably without time stamps). Posting #8 in https://forum.openstreetmap.org/viewtopic.php?id=23912 looks like it names an application that might do this. Most of this thread refers to routes that are officially in OSM, and we don’t want people to upload personal mapping. However, post 8 seems like it might allow offline relations to be converted.
I would note that umap allows the creation of custom maps, but I don’t if you can export them as GPX, and I can’t remember if they are snapped to the OSM features, or just drawn with OSM as a backdrop.