in Niger there are the air mountains, you might especially like the east side where the sand dunes interfere with the rocks.
For mountain scenery you have to visit the Ahaggar mountains in Algeria (Hoggar).
Very nice scenery also: Garet El Djenoun at northern Teffedest.
Beautifull also the Immidir plateau near Arak.
To obtain a visa for Algeria you probably will need an invitation from an Algerian Agency (or a booked tour elsewere).
If you are lucky to obtain a visa without, it is still possible that you will need a guide to move.