Is it acceptable to ask a professor who is not your supervisor to give you M.S. thesis topic? I'm currently a M.S. student. My supervisor give me freedom to choose thesis topic. Since it is so hard for me, I decide to ask another professors whom I consider to be their PhD candidates in future. I have written an email to ask them. 
Is it a good idea to do so? And is my email appropriate for this purpose?


  Dear Professor xxx,
  
  I am a M.S. student at University of xxx-the most prestigious university in xxx- with a major in information technology- multimedia systems. I will graduate on September 2016.
  I had the chance to read your publications and some of your articles. I really enjoyed them, and it gave me many ideas for my future research. Due to my strong interest in your research area and considering the experience I have gained from my relevant courses, I believe I can be a beneficial part of your research group, if I get a chance. 
  Since I haven't chosen the topic of my M.S. Thesis, I prefer to ask you to give me some information about the topics you are currently working on, in order to get a higher chance to be your PhD student in future.
  I know you’re very busy so I appreciate any time you can give me.
  
  Thanks very much,
  Sincerely,
  xxx
I occasionally get emails along these lines, and I am almost never thrilled. The sole exception where I would consider this case is if I know (a) the student personally and think (s)he is good, and/or (b) I know the advisor personally, and (s)he recommended the student this model. In all other cases, my response would be negative.

What you need to be aware of is that a master student who is actually somebody else's student, and who presumably even sits physically remote, is a risk on so many levels. You have no idea how good the person is. You have no idea whether (s)he actually works or is just procrastinating between status updates. You have basically no leverage, so if you assume the student is not working hard or is involved in unethical behavior (e.g., rigging the data), you can do pretty much nothing except pull out of the cooperation. You have no idea whether the person's actual advisor is fine with this, or will start sabotaging the agreement when (s)he figures out that her/his student is actually working with somebody else. It is even unclear what the implications for the student are - maybe her/his university does not even accept a thesis primarily supervised by somebody outside of the institution. And even if all goes well, you are still supervising a student in somebody else's name (i.e., doing the work of somebody else).

All these things considered - would you invest one of your good ideas, or, even worse, your limited time supervising such a student?