If you want my advice (do you?), instead of coming up with grand ideas, focus on planning everything out (gameplay, mechanics, implementation) and then start coding it. If you keep adding to the concept, youll never get anywhere. Get the basics done first - level generation, moving around, fighting, death/winning etc. When you're done with that, if you still have time, you can start adding fancy features, but no earlier.
Mind you, ASCII roguelike won't impress anyone, and sprites/textures/models are time consuming. Unless its supposed to be something basic and not really meant to impress, in which case this is all beyond the point.
Unfortunately I am not aloud to use external libraries other than the .Net framework
And how are you gonna do graphics without external libraries? Or is it console-only, or, gods forbid, GDI?
Do you plan to have the game as a side-scoller or from a top-down perspective?
Have you ever heard of sidescrolling roguelikes, I sure as hell have not, how would one play such a roguelike.
Uh, Spelunky? Ive seen some others, but can't remember the names right now. And i don't see whats the problem here, i can easily imagine such a roguelike. You move left, right, can jump up or fall down, climb ladders, stairs etc... other than that, not much different from classic top-down, really.