Hmm. for me, "republic" and "senate" are words and concepts I just can't mix into my Tekumel. Where did the ideas come from? They imply an approach to political power that you'd have to rewrite too much of Tekumel to implement (imho of course). But...
spin it as a conspiracy that is not quite so open, and I could work with it. I think I'd take inspiration from Chinese history.
http://en.wikipedia.org/wiki/Category:C ... rebellions
Depending on who you set as emperor, some of the Very High clans could be likely rebels: Sea Blue is arrogant, wealthy, and claims to descend from the royal house of the First Empire; Vriddi and Ito are both notorious separatist groups. There would probably be a religious component to the rebellion too, so pick a temple (the change temples seem more likely) and spin a fanatic cult for it.