I can't think of anything for Number 13, except something ridiculous like Platinum, or a fictional material we haven't seen in a previous Elder Scrolls game.
Forget perks. Uncovering the lost crafting secrets of the dwarves should be its own quest.
I actually wrote a tl:dr suggestion post about something like this back in '08 before the forum reform. Adding a great level of value to exploration by getting the crafter out of the smithy, and into the world to discover techniques was something I really wanted to see done, and this was before we even knew crafting (or even that the next game was Skyrim) was going to be in. So yeah, I really hope something like this happens.
The same could be said for all crafting, and even some of the magics. Though rare, finding stuff like the Secrets of the Dwemer Animaculti (spelling?) was great, even if it wasn't particularly useful.