Not at launch. There is research in this game and that includes researching government types so perhaps in the future such things will be possible. Even then a lone town is going to be at a serious disadvantage as they do not have the support. So even as things develop I think it will be far more likely to see your town become part of a republic, oligarchy or theocracy rather than purely independent. Just like in our own world towns going it alone are rather unheard of, they are part of a larger country with its own ruling ideals.
Also as a Barony is made and paid for by a Duke there is no way to have one outside a kingdom even with research though there might be alternatives.