Hello,
I think that looking into main scholarly journals should help to find vibrant topics.
For instance, you could consider a research perspective of a journal or conference paper dealing with your subject of interest. Discuss any PhD subject idea with supervisor and peers, it may help a lots, they could tell you about the value of a subject you selected and give confidence you lack to start.
I have spent two years to find a subject that interest me and that I am enthusiastic about. Actually, I was before that working on a different subject proposed by ex-supervisor and I was not enthusiastic about it at all.
My field is Computer Science, maybe your field is different, nonetheless, you lose nothing by giving a try :-)
Good luck.