Etiquette of publishing (continued) research after graduation with an MS degree?
The big issue here is the following: will you need to have your master's thesis advisor write you a letter of recommendation in the near future? If so, then it will be potentially a big problem to publish material from your thesis without his involvement. This will provoke a response, which could be damaging to, for instance, job applications or graduate school admissions. So tread carefully before trying to publish the MS work.

However, since your advisor has proscribed work in other directions as part of your thesis, you could certainly explore those on your own. In such a case—since it isn't an outgrowth of your thesis work, but essentially a "parallel" work, you wouldn't need to share co-author credit with him. This is especially true if you write the paper and direct the research yourself; in such cases, he has no real claim to being on the paper.