As there a some project in Telvanni Isles, I wanna drop some my ideas to add content to this place, because at now this is pretty empty.
People in Bahrammu village can tell you about old story about voung villager who left this place and go to Port Tellvanis, because he wanna become a mage. For a lot years he serve in Great House Telvanni, and when he become a wizard, he left main Telvanni island. There are rumors, that after that he go back to island and build stronghold there, but no one know where it is.
At north part of Island you can find a strange cave. Around entrance from earth grows green tentacle-like things that grow around telvanni tovers. Main entrance is typical cave. After you go in, you can not leave - door is enchanted to not open.
In interior after 2-3 parts regular cave there are giant cave-hall, like in Helnim diamond mine, there a lot small regular cave mushrooms and in the middle, on undeground river and lake - big telvanni tower.
Inside pretty empty - a lot interior stuff, but no NPCs, exept kitchen. Here you can find argonian female. At first time she will be scary when see you, but very fart clam down. He tell you, that you are in trap - master of this tower is total recluse and never let someone out. When you ask her about who lives here, she answer that only she and master of tower. If you ask she about how to escape, she answer that there probably a way through undehround lake, where some underwater caves. She also told that she good swimer, but this lake full of giant slaughterfishes, result of one of experements of Master, and she very fear go close to lake (player can escape from here every time he want if he think that he powerfull enough to kill all monsters and swim across underwater labyrinth. After that you can back to cave and tower, say to argonian that path is clear and she can escape. For that she give you soulgem that she stole from Master and after you leave tower dissapear from it). Also she can tell story how she got here: she escape from slave ship something like year ago, swim to this island and go to this cave in hope to hide and was traped here. Exept she and Master there was khajiit old slave, who cooked to Master before she arrived, and he dead some month ago (player can find his poor grave in cave). She also can say you that you can talk with Master, who lives upper (there stairs, no levitation needed).
Master - old dunmer who slovely become blind and he totally cold-heart and apathetic. His cave he call Innier World, and he never move from here for years. When he see player first time, he will laugh at you that you in his trap, but he say that he no need new slave and he will relise you, but problem that he forgot how to do it, so you need go to the library, than on lower level and return with his notes. In library you can find some regular books, some scrolls. But problem is that there Fire Atronach and he burned some books. Notes of Master is half burned. When you back with notes to Master, he remembered, that he summon som Atronach to protect library and forgot about it because he never visit library for years.
To help him remember burned parts, you need to read notes and give him things that was mentioned here (his khajiit slave - khajiit scull near his grave, etc.).
After that he ask you to give him powerfull soulgem. He advice to ask his slave, because she had better memory. If player has no soulgem in inventary, he can ask argonian: she give it you make her escape, or if your disposition more than 70, or you can kill you, Master will not care about she.
When you give soulgem to master, he will create scroll to open door for you and teach spell how to open magically-closed doors.
Additional quest: if argonian dead/escape, he will ask to pick mushrooms inside cave and make him soup to eat in kitchen.
If you finish quest about door but not kill/help escape argonian, after some days you visit this tower you find that she kill master and then kill herself. You can also find a letter from her that she can't ljve like that any longer.
Edit: after finishing post I relised that this is become too crazy :D. It actually can be more simple - like just telvanni guy living in tower undeground. And magical locked doors - like they was in daggerfall. TR can add them and spells that open them, it will be awesome if it possible