Delving in open areas of research can be good. You may want to think about a topic you are interested in before diving into the literature. You may come up with ideas that you would not after you have read the literature and seen how problems have been solved. Don't ignore what other people have done. In our department, this is something that is very flexible.
Some people give their proposals very late in the process and it is almost their defense .
Also, your thesis does not necessarily have to satisfy your original proposal. Basically, when to do your proposal is something to decide with your advisor. You should make sure you have all the signatures you need and an outline of your plan for research. Students caution not to do your proposal too early, you should have some partial results first. Don't necessarily run away if you get stuck.