Ummm, a PhD is about original contribution. Implementing a "feature" that someone else imagines is about algorithm implementation. An original contribution is about recognizing a hard problem and at least coming up with a novel solution. Ideally, the candidate also provides a thorough description of the problem in the language of her/his discipline. For the most part, if someone can describe a "feature" they'd like to see for LTtng, then they know the problem they want to solve. Defining the problem is often most of the work; once you know what you want to do, the rest is easy. Of course, there are problems that are known that don't have solutions, or at least not optimal solutions.
I'd say the poster is better off (and hopefully already doing this) working to become an expert (aka PhD) on the topic, say, by joining the ACM (maybe IEEE, but since they share databases in this area maybe not), and entering the terms "linux unix trace" into the ACM's digital library. I did that and got all kinds of interesting papers and potential topics, none of which are mentioned in this thread.
If soliciting thoughts from /. was enough to get one started on a PhD, then perhaps we ought have a /. category of Ph/.D. ;-)