Agreed, I would love to add a waterfall to the sanctuary. The secret entrance isn't even as epic as the thieves' guild secret entrance. I just can't wait to see what they add on for free.
Personally, I'd love to see a Morag Tong storyline. Eliminating the Dark Brotherhood and restoring the Thieves Guild back to it's former glory could allow this to work because you will find a new recruit who was once from the Morag Tong. He has stated that he'd one day wish for the guild to come back one day. That being said, I think it would make sense for the player and Ravyn to build the Morag Tong from scratch. This will be accomplished by recruiting new members, paying for the new headquarters to be built, and establishing connections with each of the Jarls by eliminating anybody that they feel is a potential threat(on the condition that you have NOT started the Civil War Quest).
Once all this is accomplished, this is where the real storyline begins:
Mephala speaks to you and tells you that Ravyn should be avoided. You don't acknowledge this warning because you think you're just hearing voices. Ravyn then sends you on a mission where you have to meet up with any one of the Jarl's men. The option of fast traveling and/or riding on horse carriage must not be allowed during this mission, just like how the fast travel system is temporarily disabled right after assassinating Vittoria Vici. When you pass by a certain building, a number of people who are dressed in Dark Brotherhood ambush you. As the badass you are supposed to be, you easily kill them off and loot one of their bodies to see if anything can be revealed about why there are Dark Brotherhood members again.
It all becomes clear once you read the note. You visit the Shrine of Mephala which is close to the guild's sanctuary. She speaks to you and reveals that you must ask Byrnjolf about Ravyn's whereabouts. He mentions a location that's spoken in a riddle of sorts, just like how you figure out where Karliah is after interrogating Gulum-Ei. Mephala reveals to you where he would be based on what is said. Once you kill Ravyn, everybody else in the guild seems to despise you to the point where they will not accept a contract.
Mephala rewards you for getting rid of Ravyn, but has one last major thing in store for you. Since she acknowledges that you are shunned by the very men and women you recruited, you are to pay a visit to Ulfric Stormcloak for a special request. He asks for you to hold the leader of the Aldmeri Dominion hostage and threaten the elves to end his/her life if they do not nullify the effects of the White-Gold Concordat. After they do this, you are to wait several days until they announce to all of Tamriel that this is official. Then you are to officially assassinate the very person you threatened to kill.
Not only will this allow peace between the Imperial Legion and the Stormcloaks, but you will have officially re-established the reputation of the Morag Tong. Also, you will have regained the respect of your guild members. All that would be left to do are miscellaneous quests, which would be writs of assassination that are from any of the Jarls.