You always may end up with Tikalite UFOs roaming about your space.  (The Tikalite aren't just the Silent Imperium (Or simply the Silent Pact- Silent Imperium sounds more like a term used by the propoganda of their enemies, doesn't it?), they also are a NPC generic for traders and such).

Also, is it just me or do the New Romans and Eternal Union have very similar technologies despite one of them being a alien empire and the other being a human faction?