you just need gemtext. Bold/Italics can be done with unicode, no need for a markup language.

Images can just be links and it would be a setting on the client to open it or not. Like what the Lagrange gemini browser does: it lets you click on a link to an image to load it.

I would argue that even tables are superfluous, you could put a csv file in a block quote and people's clients could just render it optionnally.