I returned the Skeleton Key.

So that's it? I am now Guildmaster? There are no quests prompting me, about the Thieves Guild, at all. Feels like a let down. No one wants to talk to me about anything except for fencing my stolen stuff, training and the assignments.

Mercer's bed is still owned.

Is there safe storage in the Cistern?

Are you sure you have finished "http://www.uesp.net/wiki/Skyrim:Darkness_Returns"?

No you're not Guildmaster until the ceremony takes place. The "special" assignments in the four cities need to be completed before that happens.