I have a suggestion for a couple of new tags (one should be easy, the others might be harder):
<pgbreak> - forces a new page like <chapter> does, without making an entry in the TOC (would be useful for title pages, among other things)
<u>
underline</u>
<monospace> or <mono> - force the text into a monospace (courier-ish) font (i dunno if the reader has one built-in, offhand) and have all spaces displayed (would be useful for tables, and things of that nature)
and maybe inline images, somehow?