I don't think a flashbang would work that great, but a concussive/stun grenade could, maybe. Something that works like the Arc Star stun effect. Very short fuse (unable to be thrown further than maybe 15 meters before detonating), deals 5 damage (purely to see if you hit someone with it). I'd see it more as a long CD tactical ability for a legend rather than a new grenade though. Being able to spam hard to dodge stun effects would be really annoying to play against. I think it'd still be rather annoying to play against even if you could only use it once per fight.
A stun gun ability could work better. Like... A limited range taser that does no damage but applies the electric stun/slow effect. Think, uhh... Ana's sleep dart from Overwatch. Press button, whips out taser and fires a projectile automatically, but with a hard limit on range of around 10-15m.
I think I'd personally prefer not getting more AoE CC items though, and for AoE stun ability, technically Crypto already got that covered with his drone. Maybe buff him to make it easier to use EMP instead of adding new grenades?