I'm a cartographer specialized in trekking/hiking maps in scale from 1:75.000 to 1:25.000 for regions without any accessible base maps. Now, I'm looking for new work for the next projects :) Perhaps anyone here is missing a map for a certain hiking trail?
Of course, there are many long-distance hiking trails without any available maps.
As far as I'm aware, none of the European long-distance trails have dedicated end-to-end maps, unless you count Openstreetmap or a collection of several hundred topographic maps. In some places they're well-mapped, e.g. when passing through Switzerland, Germany, or France, but in other countries they aren't, such as through Italy or Greece.
I'm sure there's plenty of traditional long-distance trails in Africa or Latin America that don't have accessible maps either.
I don't know if North America is any better. Are there dedicated maps for the Alexander MacKenzie Heritage Trail? I've found that in North America, topographic maps often don't show the trails at all. So there's certainly a market niche right there.
Except that almost nobody goes hiking there, so it certainly isn't profitable. Which may just be why there aren't any maps available.
Edit: One popular trail that appears to not have any maps available (at least not in English) is Jeju Olle. It's very scenic, not very remote, and attracts lots of hikers. It appears that all hikers simply use a leaflet along with signposts. Other trails in Korea might have the same pattern. Topographic maps exist, but are very hard to obtain.
The Kungsleden (lit. kings way in Swedish) is a 440 km long trail in northern Sweden/Scandinavia.
The trail is separated in four portions which each represent approximately one week of hiking. The most practiced part is by far the northernmost, between Abisko and Kebnekaise. The season, when the huts are open usually runs between mid-June and the end of September, rowing boats are usually at place in the end of June or beginning of July.
Huts have been constructed along the trail, separated by a distance that a walker could expect to cover during the day, about 9-22 kilometres (6-14 mi). The huts are primarily operated by STF. For a small fee, it is possible to pitch a tent outside and use the facilities. Supplies can be bought in some of them (Alesjaure and Kebnekaise Fjällstation for example). Some emergency shelters can also be found on the route.
While there are the official Swedish maps, the only available scale I know of is 1:100'000 - quite large for hiking maps. On the other hand this also prevents the avid trekker to carry an entire backpack full of 1:25'000 maps.
Especially the northernmost section (1-2 weeks) is fairly popular, and there might be a fair demand for dedicated trail maps for this section. The guides that I own/saw personally only include very crude and essentially useless maps for tiny sections of the trail.
Apart from that it is also a wonderful trail. Highly recomend it. :)