Your purpose is rather obscure. Here’s how I understand it.
You have a source (meaning “at the origin of the work flow”) file containing your document as a sequence of characters (usually a .txt file). In this basic document, you want to add some markup for turning runs of chars bold, italic or other formatting. This is equivalent to AskLibO markup where double asterisks ** mean “make next run bold until another marker”.
You can imagine many variants of such a markup, but as it becomes more sophisticated, more and more characters are removed from common use for the “in-band signalling” of the markup unless they are escaped.
The first real difficulty arises when you try to import such a file into Writer or Word: you need a filter to interpret the markup and translate it into the document processor internals.
You can opt for an XML markup. The advantage is “out-of-band signalling”: only < needs to be escaped in text. This is precisely what ODF is based upon and you can have a look at it when saving documents as .fodt.
Writer uses natively this format (.odt is a zip-compressed variant of it) and has the filter to handle it. Word claims it can process ODF but does not implement the whole standard and adds divergent features to it.
The answer to your question might be a .fodt file. It is a text file with XML markup, consequently it is transportable to any platform, albeit a bit fat because of no compression. However, the destination processor needs to have an input filter for it.
In the end, an .odt file is likely to be the best trade-off.