My second army was Chaos Dwarves. Lets all talk about that one!

My second army with an army book (in 6th ed) was VC's. They are so similar yet different to Empire. You can double the amount of zombies you build by using extra bits from your state troops and free companies, and a unit of black knights is easy to convert from our own Knightly orders... Considering their close proximity (as in previously being a state) to the empire allows you to get away with these conversions pretty easily. Its also fun to have an evil army (especially if you lend your VC's to a friend to use to play, so you can get to use the WP's hatred....)
The style is very similar - maximising CR with ranks, and flank charges (with kills generated from Vampires), then using your outnumber to run them down. Movement is pretty much the same, and zombies are (generally) dead humans...
But if you want something completely different, go for an army whose core has different movement or base size....
Orcs a good army, and with the new edition around the corner, by the 7th ed box and get half an army (alternatively use the Dwarfs - which is much better value in that box IMO)
DoW are also a good army (I love the underdog - having 5000pts of DoW)
with lots of choices - easy convertibility and integration with your empire force (you can take ogres, dwarfs and marauders as well)
I am dabbling in the thought or a marauder army led by a chaos lord as (sigmar retuned) with some DoW Dwarfs... and obviously as Sigmar has been 'battling' chaos for more than 2000 yrs, most would claim this army to be evil and tainted by chaos....
Dark Elves are interesting, but a new AB would make them more appealing, and that is still a bit off.
OK are a good option as there isn't much painting involved.... however you really need to not get taken away by Ogres being the mainstay, and Gnoblars as the support, because it should operate the other way around. Gnoblars provide the static CR, Ogres are the flankers who break the enemy.
Out of all the options though, IMO the VC's or DoW are the best choices for next army...