I want to approach a certain subject for my diploma project, but unofficially I may get a bad grade for doing so. What should I do?
Your faculty presumably knows much more about the area, and if they suggest (formally or informally) that some line of inquiry isn't suitable for a thesis, I'd listen carefully. It might be that the task is just far too ambitious to really solve in reasonable time (I see that very often with my own students), or it sounds superficially interesting, but has already been solved or the solution (if found) won't be an advance. Sometimes the thema is worthwile, but the risk of not being able to complete it (or not getting results) is just too high.