Give fighter guild a new skill line, and have a silverdawn skillline that only obtainable if you join in silverdawn, which grants you a world skill line.

You can ONLY join in silverdawn if you are not vampire or werewolf, and you need to undergo some sort of trail to become a member. (a senior member can nominate a new member, just like vamp and werewolf skill lines)

Silverdawn is can be viewed as combination of silver hand and dawnguard in Skyrim, makes them the perfect candidate for a chooseable third world skill line.

It makes sense if you think about it. A new non-daedric world skill line, you no longer possess anti-daedra skills if you are somewhat daedric yourself, and ppl who want to optimize their build without become an ugly vamp can have this option