Is it feasible to take an unsure PhD topic?
This is exactly how most of the really great PhD students I've seen have all started.  Working on a problem with unknowns is a process called "research."  

Clearly, you are going to have to find some collaborators with more experience in the field than your advisor, but that's part of the process as well.  Hopefully, your advisor can assist with networking / introductions / etc.  The support of your advisor will be critical (as it is for all PhD students).

The trick is to make sure you have a backup plan and a way to convert to it if necessary.  Don't spend three years mucking around if you're not making progress.  But, spending 6 months investigating if there is something there -- that's well worth it.