Spellmaking and Enchanting. Making your own spells like it was a piece of cake destroys immersion. There should be a set number of spells with unique names that you can learn from scrolls and trainers but that's enough. If you make these spells interesting by adding a description and unique icons that's all we will ever need.
Enchanting is also gamebreaking because you could make a full set of armor enchanted with shield and then next to nothing could hurt you. It can exist as a skill instead with limits on how strong enchantments you can make.
Think most people including me would see this as dumbed down the game to fable level and below. The inability to delete spells destroys immersion as you has to reload after making and testing new spells, spellmaking and enchanting is half the fun, without them the game would be far less interesting.
Personally I don’t see Oblivion as dumbed down compared to Morrowind except from the quest compass. Yes lots of stuff was cut but changes to magic and enchanting were made to reduce obvious exploits and balance the game. In my opinion the changes was mostly good but enchanting and spellmaking was cut more than they should be, why not two cheap constant effects like detect life and water breathing on a item? Are restore fatigue 2 / second constant effect game breaking? They got lazy and cut more than they had to.
Yes you can add shield to your armor but armor rating is cut at 85% anyway and this stops you from getting other enchantments. You got the best shield effect from sigil stones anyway not by enchanting. To get any real effect you would need grand or black soulgems, hard to get grand souls at low level and black require knowledge about them.
You can make some pretty lethal weapons with custom enchantment, but they require knowledge, testing and have very few charges.
Yes you can make a chameleon suit but that is pretty boring.