Well, I never played T4, but I always thought that there would be several rivals to the infant 3I as it grew. Ilesh should have had a good sized PE in existence. Perhaps several good sized rivals from the remants of the Vilani Empire, none as big as the one around Vland, but still decent sized.
M0 assumed the 3I was only a sector or so in size right? Figure any of the DOMAINS were probably based on adding large PEs to the 3I as it grew. Do that and you can place several rivals from that.
You probably shouldn't have more than 1 or 2 large rivals, otherwise it wouldn't develope into the Sylean Third Imperium, it might become the Ileshi Third Imperium with the Sylean Domain.. Can't fight a war on too many fronts and expect to win. Lots of little, sub-sector sized powers yes, big Sector-sized powers, probably not.