So now you've completed everything that the Dark Brotherhood quest-line in The Elder Scrolls V: Skyrim has to offer. But now what's left to do? Anything? Well sonny, there is something you could do! After you complete the quest-line, one of your goons will suggest to you that some upgrades are in order for the sanctuary. Doing so will add some really cool bonuses.
YouTube user pms00 has another great guide detailing how to get each upgrade, and what they do. Hope you have enough gold to get everything!