This is a suggestion I put on the subreddit a few days ago, but I figured I should post it here as well and hear your thoughts.
In the Imperial City, in the location of the former Mage's Guild, there should be an artifact that allows a rudimentary spell-crafting system similar to the one found in Oblivion, perhaps attached to a quest that must be completed in order to use it. I am no expert in the architecture of Skyrim's engine, but it has been done before by multiple modders in fact, and one of them might be able to lend their advice or loan their code if we do not have enough resources to dedicate to it. In addition, we have the previous games for reference as to how to balance it.
In order to make these new locations feel like a natural part of the game, adding a new, unique, location-specific system like this would go a long way and I think the team (if you're reading this) should really consider it.
As an example for a quest to attach it to, I think attaching it to a major Thieves' Guild quest would be a very fitting choice, with the background that after the dissolution of the Mage's Guild the guild properties either fell into the hands of the government or were auctioned off to some wealthy noble. Alternatively, it could be done as a quest for some other magic organization (Synods?) where the player is asked to travel to the Imperial City and procure the artifact by any means necessary. Keep in mind these are all just examples of how it could be implemented.
Adds tons of fun and customization to the Skyrim experience.
Brings back an old feature that everyone loved.
Entirely lore friendly and fitting to the location.
Can be seamlessly integrated into a guild quest or main quest.
Potentially has most of the groundwork laid by modders already.
Seriously, people will spend hours messing with this stuff.