Well, the main thing is that they'll be readable and writeable both by the user interactively and by the program. I'd like the program to see the text as just text, not cluttered with html or whatever, even if there's html in the implementation. I see a need for two kinds of interaction. One is the text editor style, in which there's a body of text that you jump around and add to throughout the document. The other is the conversation style, in which new text is added at the end, and the stuff above there doesn't change. So if you start editing a line earlier in the document, it gets copied down to the bottom and your edits happen in the copy.
My ideas for this are based on what we implemented in Object Logo 30 years ago...
But you'll be able to choose fonts and colors and all that. I think a text box should be considered a costume, so that it can be attached to a sprite which can then move, turn, etc., around the stage.
But none of this is official! I'm guessing Jens will want something simpler, but I don't really know yet.