I feel the options in this game are too limited. As skills change, so do the viable "flavor of the month" class setups.
I don't think we can completely remove classes because of how integrated they are already with the game.
Allowing characters to add in one skill line from another class would open up the build possibilities immensely.
You could make it similar to Vampire or Werewolf, where you would gain an extra skill line via a quest or achievement. Then have a method resetting the extra skill line so you can swap it for something else (hopefully without having to reset all your skills).
I think this is much more viable than continuing to seek balance by adjusting the class lines so that they balance against each other.
1. Templars would be able to resolve their resource issues with NB siphoning or DK battle roar, or Sorc dark exchange
2. Dark Knights could gain a class ranged attack from Sorc storm calling or charge ability from NB assassination or Temp aedric spear
3. Nightblades would have access to better self healing with Temp restoring light or DK draconic power
4. Sorcerors would be able to ... well sorcs can do it all right now, but I'm sure there are still some good multi-class options for them.
Everyone gets a lot stronger and a lot more fun to play with a change like this. And there might be a possibility that a completely overpowered build could be made, so likely some minor adjustments would have to be made.