I am not an expert but rather familiar with Western Nepal as I did the Upper Mustang trek a couple of years ago. In the Western Nepal, to my knowledge, the most popular treks are:
- Upper Dolpo
- Upper Mustang
For both of the above treks there is also a "lower" trekking region. Both treks are rather expensive, at least for foreigners, as they require restricted trekking permits. Also, restricted areas in nepal can only be done by groups, meaning more then 1 trekker per tour. For more about the permits costs per day, please see the following link: http://www.taan.org.np/info_details/trekking-permit
Mustang is a great region for trekking and probably most popular for its Tibetan influence, and a trekking route filled with cultural experiences and vast barren trekking landscapes (the Tibetan Plateau). While I have not been to Upper Dolpo myself I know that much of the trekking route is also via the Tibetan plateau. Both treks are rural treks so not many tourists, compared to Annapurna or Everest region.
In terms of complication there shouldn't be much other than you might need a guide as the trekking routes are not well travelled, and also, it might be expensive (but not sure if Nepali people have to pay for trekking permits).
Perhaps the above can be of some help.
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