Agility is important for evasion and just the overall case of arriving within visual range combat, not to mention if you encounter another and equally as potent stealth aircraft, you'll be relying on IR tracking or Visual sight to find him, which is automatic visual range combat.

Durability doesn't really matter since a missile will blast any plane out of the sky, no matter the armor. That also causes a weight penalty. The less weight = more effective

Definitely make it cost effective, since then you can field more in the case scenario of losing some units if that were to happen.

Other than that, the aircraft could be multi-role, adds on to cost-effectiveness, makes it less of a logistical nightmare, and less unit complexity.

Then it just comes down to whatever mission you need the aircraft to perform generally

BVR capability? Probably even more important than snap maneuverability.
At least some level of stealth.

Extremely tough

I think survivability would be more important, which you mentioned. Better to not get hit at all when increasing armor enough to reliably stop modern weapon systems decreases performance.

Deployable in mass

As long as it doesn't compromise the quality of a single aircraft, I guess. But, generally, less is more. (Do more with less units)
Of course, I'm no expert, so take that with a boulder of salt.
Oh, and ability to find and secure WMD's, AKA oil.

You forgot to add a waffle maker

Pros:
Waffles

Cons:
Only makes waffles

@ReinMcDeer @MarbleXLazuli @Ultralight