Anyone else worried that their proposal/subject area might be too narrow for getting an academic job in the future?

Yes, I am very interested in PhD work, however I am concerned for the job prospects afterwards.

I want to do something in a relatively new field, but looking at job ads for current academic jobs, my interests don't really apply to any of the job descriptions. Should I do something completely mainstream???

Be original - that's the exciting part of doing a PhD! You never know your area of study could become popular in the future.