This is an idea that I've had a long time. Why couldn't they give us a new 'Class' to learn in Cadwell's Silver, and then a second new 'class' in Cadwell's gold. Essentially this would allow players to be 3 classes once they have completed the storyline, but they'd still need the skill points to pull it off. It would also mean that you would not have access to the fourth class skill line. This would offer the power diversity that feels reminiscent of traditional TES games, and with the addition of Spellcrafting it would really open up a lot of combinations for players.
Imagine for a moment that you start the game as a Nightblade, and when you enter Cadwell's Silver you decide you want to be a Sorcerer, and finally at Cadwell's Gold you choose to be a Templar. At this point you would have access to a large number of passives of course, at the cost of a lot of skill points. Build diversity would increase, and players would be able to adapt with the tides of various game balances. It would also mean that for the player who is limited for time they do not need to level up 8 different characters to try the 8 different builds: Stam/Magicka * 4 Classes. I do realize they would need to adjust a few things within the classes to balance this, but really what more would need doing to differentiate from what they'd need to do with spellcrafting anyway?
US/AD - Dodge Hopper - Vet Imperial Templar | US/AD - Goj-ei-Raj - Vet Argonian Nightblade
US/AD - Arondonimo - Vet Altmer Sorcerer | US/AD - Azumarax - Vet Dunmer Dragon Knight
US/AD - Barkan al-Sheharesh - Vet Redguard Dragon Knight | US/AD - Aelus Vortavoriil - Vet Altmer Templar
US/AD - Shirari Qa'Dar - Vet Khajiit Nightblade | US/AD - Ndvari Mzunchvolenthumz - Vet Bosmer Nightblade
US/EP - Yngmar - Vet Nord Dragon Knight | US/EP - Reloth Ur Fyr - Vet Dunmer Sorcerer
US/DC - Muiredeach - Vet Breton Sorcerer | US/DC - Nachtrabe - Vet Orc Nightblade
EU/DC - Dragol gro-Unglak - Vet Orc Dragon Knight | EU/DC - Targan al-Barkan - Vet Redguard Templar
EU/DC - Wuthmir - Vet Nord Sorcerer | EU/DC - Kosh Ragotoro - Vet Khajiit Nightblade
<And plenty more>