I ran across a blog post the other day (which I cant seem to find now despite hours of searching), that discussed the concept of making a game with only one class! What an excellent idea! This has already been done to some degree, mostly centered around the rogue-type with games such as Thief.
Not only would it be interesting to delve into such a focused area to try and make it engaging, interesting, varied and fun, but I could see doing a small focused game along these lines as a prelude to producing a bigger game that combines the concepts gameplay from the smaller games and incorporates them into a whole.
Click the link below for some random thoughts about what could be done with this idea.
Rangers - Game would be mostly outdoors. Rugged wilderness with lots of nooks and crannies. Mostly small villages who would be somewhat distrustful of the wornout stranger walking into their town. Villages would just be for some light trading before the players would set back out into the wilderness. Gameplay would be centered round tracking, outdoor survival, learning the lay of the land, etc. Opportunities for combat, puzzle solving and exploration abound!
Soldier - This game would start you off as a guard in a town. You do patrols, break up bar fights, gamble and drink on your time off and eventually work your way up to joining the army where you are sent off to fight battles in which you find yourself just one of dozens or hundreds of combatants, and your main focus is to defeat the guy in front of you and keep an eye on whats happening in your general vicinity while trying to maintain your formation and respond to any new orders from the leaders. Maybe even moving from this closeup individual combat to commanding your own squads and armies!
Mage - Entire game based inside an impossibly huge tower that likely crosses and exists in several dimensions at once. The basic concept is that the player is a new apprentice who must learn to use magic, pass tests of their magical abilities, and explore the mysterious tower without getting caught trespassing into forbidden areas while also avoiding being eaten or imprisoned by various entities who live in the recesses of the mammoth structure. There would also be various factions within the school that the player can choose to join which would further alter their playing experience as they advance through the ranks. This would also be an excellent place to try out a complex spellcrafting system!
Priest - Player starts as a novice priest dedicated to a certain god. The goal is to increase their gods in-game presence through converting heathens, making appropriate sacrifices and establishing new temples.. As they gain favor, the player can gain new powers. They'll even have opportunities to change the deity they are aligned with... for a cost. Avatars for other deities would be also trying to promote their gods and as such would cross paths with other players, sometimes in battle, sometimes in uneasy alliance.
Just a few examples off the top of my head. In some ways, having such a narrow focus provides even greater opportunities for creativity than a more open game would.