The areas starting to really get my interest are a bit far from my superviser's area (and there is no real 'expertise' in this area within the rest of the department) but I feel that what I did on my original proposal just doesn't hold any interest for me anymore. Should I stick with what the people in the department can offer me, or risk doing something a little different without the expertise to supervise?

that's tricky.. usually a PhD needs quite many discussion and supervision.. how about consult your supervisor about it?