orichalcum

I don't know... were the Tribunes actually typically controlled by Senatorial factions? If not, then I would suggest that you reserve that as part of the "gameworld" that you control. If they want to get a veto, they can engage in demagoguery to try to accomplish their goals. And if they do something that the Tribunes would veto (in your judgment), then they face a veto and have to either bring the Tribunes around or deal.

The danger is it being a mechanic which is fun and interesting but bears little resemblance to the actual way Roman politics worked. (If the Tribunes were typically in the pocket of one faction or another, then never mind.)

--Adam