Windsweep Jaffer?

I'd recommend waiting until you know what the upgrade does before finalizing your build.

But I don't think it's a good idea, since Windsweep just stops Jaffar from doubling. That's not worth it especially since half the time he'll be attacking sword/lance/axe users that won't counter him.

Savage blow is a good idea, since it will stack with the built-in poison strike on his weapon, but you should definitely wait until you find out how his weapon can be changed to do anything about skills.

The best b-skill for Jaffar is quick riposte: he has HP to tank 1 hit from brave Lyn and enough speed with life and death to avoid being doubled. His low attack doesn't matter against her horrible defense.

You should wait. To be honest, Jaffar’s attack is too low to kill in one turn so there is no build right now that can turn him into a ruthless killer. He is more of a chipper/debuffer, so I think windsweep is one of the best options for him. However, I have the feeling that the upgrade could be windsweep, a great attack boost, some kind of flashing blade effect, or an AoE debuff that will make him way way better. Wait until we know how he will be boosted, then you can decide if windsweep is the best option for him.

While the prospect of Jaffar doing 14 in AoE damage from Savage Blow and getting free chip damage against DC units sounds very entertaining as it is, I have to go with the others and suggest waiting a few days until the update rolls around. Don't jump the gun until you know exactly what the upgrade to his daggers is going to do.