You should just make the type of mods that you would like to see in the game. It doesn't matter if they've been done before, options to choose from are always good.
The best way to know what might be lacking, is to start playing/investigating mods and learning what works and what doesn't.
http://www.yacoby.net/es/forum/12/6543311176714780.html still has lots of ideas in it.
Pretty much this. Create something that you feel would be fun to make, and if others want it too, all the better. I doubt very many modders have created things without wanting them personally.
2. The mods Rise of House Telvanni, Uvirith's Legacy, Building Up Uvirith's Legacy and LGNPC Tel Uvirith combine to make being the head of House Telvanni an incredibly immersive experience. With these, becoming Archmagister is just the start of your career with the House, not the end. I'd like to see similar mods available for the other two Houses. Someone is http://www.gamesas.com/index.php?/topic/1106876-early-wipz-house-redoran-expanded/, and they're looking for help.
This is a common one, though, if you're looking for something huge that isn't Tamriel Rebuilt (or one of the other province projects)
. We have so much incredible Telvanni content that the other Houses look bare by comparison... maybe one day it will be the other way around!