Re: Why aren't there any more powerful "empires" in Federation vicinit
As for the Romulans, the only ones I know of that gave them a bloody nose were the Borg in pre-TNG - early 24th century times. I think they were pretty small and isolated other than that - certainly more so than the Klingons. Every map chart set ever produced shows the Romulans to be the smallest of the "big three" - at least during the 23rd century, and not counting groups like Tholians, Gorn, Cardassians, etc., who were all quite small, relatively speaking.
In the FASAverse and several novels they called them the "Kinshaya", or "Demon Warriors". Canonically, there had been several oblique references to an aggressive warrior race called the "Hur'q" that assaulted and possibly even enslaved portions of the Klingon civilization for centuries. After overthrowing the them, the Klingons stole their ships and advanced tech and set out to build their empire. The D-7's and K'T'Inga's were apparently based off these designs, which is why they always looked like that from the 22nd to 24th centuries and beyond.Some novels suggest the Klingons and Romulas had been fighting another species or two that were on the far side of their empires from Federation space and keeping them busy for decades at a time. For the Klingons it was supposedly the reason they didn't stop to deal with the Federation until Kirk and Pike's time. For the Romulans it was why they hadn't bothered with the Federation for nearly a century at a time twice.
As for the Romulans, the only ones I know of that gave them a bloody nose were the Borg in pre-TNG - early 24th century times. I think they were pretty small and isolated other than that - certainly more so than the Klingons. Every map chart set ever produced shows the Romulans to be the smallest of the "big three" - at least during the 23rd century, and not counting groups like Tholians, Gorn, Cardassians, etc., who were all quite small, relatively speaking.