Well as B-ZaR already suggested, Markdown could work well. It also occurs to me that we could use a subset of BBCode, which was designed for the exact purpose of providing a set of formatting codes that could be used to format user provided content without having to permit the use of raw html in the content itself. There's no need to re-invent the wheel here.
Oh, did miss that - sorry.
I am not comfortable with using "BBCode" or the mentioned "RestructuredText" as this sets the entry level higher for package maintainers as they occur to me as rather "exotic" languages. Markdown may have its merits, but which one would or should it be ? Depending on the intended requirements (= what should be possible in terms of text magic) we may need to use a variation that also allows to embed html. And then we are back at square one: if we need to filter anyway, why not html directly > its widely know, and a gazillion tutorials,editors, libraries, controls, etc are already available ?