I have a player who is using a Divination Wizard.  He is wanting to create a spell that lasts as long as mage armor, but gives disadvantage to incoming attacks.  He sees it as looking into the future and standing where the attack is not.  I can see it, but am unsure what threshold to set the spell at, and whether to have it require a save like a fear effect.  Any thiughts?

Doesn't a divination wizard already get Portent?

I'd probably not have it apply to EVERY roll targeting the wizard? Give it some sort of limit that increases with spell level.

Maybe start small, like a level 1 spell that gives you a halo of lights. Before someone rolls an attack against the wizard, they can spend a light to impose disadvantage. At 1st, get something like 1+ Int Mod lights (min 1). If you cast the spell at 2nd level, or higher, you get an additional X lights (maybe 2)? I don't know what a fair number would be, but if it lasts 8 hours, spending all lights after the second battle might feel crappy.

I probably wouldn't give it a saving throw because it doesn't sound like it targets anyone; it's more like "out in the universe."

I don't know if it exists in Exalted 3rd Ed, but in the first Sidereal book, there was a dodge charm that they could cast before being targeted by an attack at the beginning of combat. If they succeeded, they declared they actually weren't there that day and instantly teleported back to the beginning of their day. Something retroactively delayed them and they don't show up again until after combat.

Between spells like Blur, Mirror Image, moving up to Foresight, it seems kinda pointless. And the Portent feature too..

You're basically giving them a cloak of displacement, which is a wondrous item that requires attunement. Try to balance the spell based on that concept.  

I think requiring concentration isn't to much to ask for a divination spell.