What can I do, if I don't like the proposed research subject by my potential supervisor?
A successful project profits from both supervisor and candidate being interested in the same question. If you really dislike the topic, you should try to convince the supervisor to accept a different question. He will then, if he is amenable, try to convince you of a variation of his, you will amend it, etc. until - hopefully - convergence.
However, you may find that the supervisor is simply not interested in your direction. This can happen, even if he likes you. That's fine, and then you either have to find somebody closer to your question of interest or compromise.
That being said, it seems that you are talking about an MA Sc degree. This is considerably shorter than a PhD, so, depending on the details of the situation, it may be easier for you to find a compromise.
If topic really matters for you, however, you need to think hard whether you want to go into a field which you have qualms in.
I am not sure about what the ethical consideration of a "theoretical topic" as opposed to a practical one could possibly be; be is as it may, I would avoid bringing this up, as it may insinuate that you cast doubts about the ethics of your prospective supervisor.
I sometimes find it helpful to think about research focus as having three entirely distinct aspects:
- Passions: the core "gee whiz" that is what really draws you to research (e.g., in your case, swarm robotics)
- Techniques: the methods and skills that you actually use in your pursuit of a research project (e.g., discrete-event methods, domain-specific languages, control theory)
- Applications: the societal issues that (e.g., environmental issues, civil security, traffic, elder care)
There are often many different techniques that can be used to pursue the same passion, and many different applications that can be used to motivate its development.
These pieces can combine in different ways, and at different times in one's career, different elements will be on top. It's most satisfying when your passion is on top, but there are times when you need to focus on one of the other areas, e.g., to develop or apply skills in order to advance yourself to where you can get back to your passion, or to work on application-specific elements that are necessary in order to make your passionate work relevant. These may be passing detours that you need to take and return, or they may turn out to be more interesting than you thought and to lead to new passions of their own.
Where you are in your career, right now, I would suggest not being too picky about maintaining focus on your passion. If you can see ways that the other things you do might be later related back to your area of passion, then I would suggest taking that detour. You don't need to let go of your passion, and you can keep looking for ways do satisfy both elements, but right now, at the Masters level, you probably need to focus on the techniques aspect of research.
All things are negotiable... unless, conceivably, your sponsor-professor's research money is explicitly tied to a specific goal.
Also, as in the other answers and comments, it would surely be profitable to you in the larger picture to broaden your expertise in directions that your sponsor thinks would be productive. His/her experience should translate into insight and perspective that you yourself might not yet have.
And, responding to a particular point in your question and comments: I don't think that different "specialties" are as disjoint from each other as you seem to worry. Work in a "different specialty" might indeed be very helpful to you in gaining new understanding of your "current specialty", maybe seeing possibilities for innovation that you hadn't seen before, due to "being in a rut".
Also, although the immediacy of generating publishable research has its appeal and reward, some enterprises (e.g., education and acquisition of insights) have a longer cycle time.