I know of a few sources of flat damage increases, such as the Maelstrom Two-Handed Weapon "merciless charge" (post update 39) which grants up to 560 flat damage to all your direct damage attacks and draugrkin's grip from unhallowed grave which grants up to 330 flat damage done for all attacks.
And there is a debuff variation (stagger debuff exclusive to dragonknights for now.) which stacks up to 3 times and causing the target to take 195 extra flat damage from all sources, which is different application wise.
But what I ask, is where does the flat damage get added in? does it get added in "before" percentage buffs and other factors, or "after"?
I ask this because if it is added in "before" buffs and percentage increases, it turns out to be somewhat detrimental on abilities that deal high direct damage, not so much for DoT abilities and effects as damage each tick tends to be low when it's overall damage is divided by the number of ticks dealt over time.
If you see me anywhere. Know that I am sitting back with a bag of popcorn, watching as ESO burns the goodwill of its player base with practices that only disrespects the players time like it did to me and many others...
If a game does not respect your time, best thing to do is move on from it and find something else.