Eliyaz
I am worried by your restriction to digital electronics. The whole point of LabVIEW is to make it easier to connect a computer to real world, which in many, many cases is sadly and obstinately analogue in nature. I would do as Bob suggests, talk with your Professors and with the Chairman of your Department. You could also ask around to see if anyone has something that they need instrumenting and/or controlling.
However I think it could be hard to satisfy the academic rigour requirements of a PhD just by doing this.
One possible topic: Find every possible method a LabVIEW programme on computer A can communicate with another programme on computer B. Understand each one, and benchmark for speed, amount of data, latency, consistency and reliability.
Or you could get LabVIEW to work with the OpenCV open source imaging library.
