But also "shop around": Select at least 3-5 themes that interest you, from at least 2-3 different supervisors (the last years 50-60 students/year have been interested in some of my projects, but I can only supervise around 10/year...)
Note: The Master's Thesis is your chance to work on something that you decide for a whole year. So take it as an opportunity to do that! And thus, make sure to pick a subject that really interests you.
So: contact Björn, so we can discuss the opportunities.
However, a master thesis supervised by me would definitely not be complete unless it included a working prototype system.
And, the thesis itself (and hence the project) should be at a level where it would be feasible to publish the results at an international conference or workshop - so writing a scientific paper based on the thesis is a clear goal (if successful, this would normally entail for the student to travel to, and present the work at, an international meeting).
Note though that there is no requirement that the thesis work should be submitted to any conference, only that the quality of the work should be at that level.