Aside from Civ 5 and ruling the world as Alexander or Augustus or Casimir, I have fallen for a rogue-like called Dungeons of Dredmor.

Reasons to play it:

Rogue-like!
You get to pick a bunch of skill trees including:!
Emomancy!
Necronomiconomics!
Mathemagics!
Killer Vegan!

I still haven't beaten it and I'm like 60 hours in. It's easy to play a bit and come back to; it's complicated; and it's cooky.

Plus, it'll be on sale for like 4 bucks total come the next steam sale, so keep your eyes peeled.