Personally, i disagree with any claim that there are more than one aikido traditions. There is only one aikido, although different aikidoka, have different personal styles in the way they execute the techniques. For example, in an other thread i had a conversation about how different is the current Doshu's personal style than Sensei Steven Seagal's. These personal styles however, do not constitute different aikido traditions. So in my opinion what Mr. Tohei, Mr. Shioda and others did, claiming their own styles is wrong, regardless of their effectiveness.
So who may open a new style / M.A. in your opinion?
Who decides if not the person himself?
In my opinion - a new style is bourne once a teacher decides his way is different than the way of his teacher. He may then open a new style of the same art or a totally new art - it is up to him as the founder to decide. Then, others must decide if they wish to study this new art/style.
the change of the new way from it's predecessor may be minor, it could be some additional techniques, it could be changes in the curriculum and teaching, could be in philosophy or any other issue. It does not matter, once a person decides he is a founder of a new way, and no longer follows his teachers - that is it.