Reception of a made, derricksburg both contain good empire cards.
I would also add the full size expansion March of the Damned because it adds several decent Empire cards like Rodrick's Raiders and Manaan Take You. This expansion also adds the lizardmen which can work well with the Empire. I would also look at The Fall Karak Grmiaz because it offers the powerful Wilhelm of the Osterknacht and the useful Tactic Called Back. That pack also has long winter, which is a neutral tactic that works well with Empire decks.
The current battle pack cycle, the Capital cycle, has each pack focusing on one of the factions. The Empire focused pack is still coming but when it does you will probably be interested in that as well.