Well...if you go off my info, you should investigate Varys, Sonata, Thoughts, or Reisen.
Based on your info, I can conclude that you, myself, H.A.N., and Hibikaze are townies (unless one is Godfather). So odds are the next person you investigate is Mafia. Whoever is protecting you should continue to do so because it's the only chance we have to find the truth.
You can also investigate me, but your odds of finding Mafia are 75% by just picking one of the four mentioned above.
I guess we can vote on someone though.
I am just throwing a name out, but I'm going with...
vote// Varys the Galvantula
and for the Detective, investigate// Reisen