Hi Tessifier!
What an excellent question! I am just applying to do a PhD in European linguistics so I have had to choose my own topic and design my own proposal, which is great I think. This way, you know enough about your area to make an informed decision about studying further.
As far as choosing my topic, during my first degree I spent a good deal of time talking to my lecturer who mirrored my interests and she came up with many interesting ideas. I then read up on the ones which really interested me. I also went to a conference she organised, which inspired me even more, and here I am: about to submit an application.
The best of luck! :)