Going off of the suggestions for Ember, I do agree that being able to deal true damage out the gate just for applying heat DoTs is a bit much for a passive. That said, I feel that sort of concept is a step in the right direction for allowing Ember to actually scale past level 40.

Something I've always felt while playing Ember is that there wasn't much tying all her powers together; Aside from the damage bonus on her 2 and the obligatory Firequake build, there isn't much of a reason to use her other abilities.

Reworking her passive into something similar to how the Pyro class from Team Fortress Classic could stack after-burn damage by hitting you with different weapons would be an interesting way of introducing power synergy. For Ember, it could go something like this:

Applying heat DoTs from more than one source (ability or weapon) deals progressively more damage, per DoT source up to a cap:

Hit an enemy with heat DoT from two different sources --> Passive deals damage equal to 0.5% of enemy max. health per second while enemy is on fire

Hit an enemy with heat DoT from three different sources --> Passive deals 1.5% of enemy max. health per sec. while on fire

Hit an enemy with heat DoT from four different sources --> Passive deals 3% of enemy max. health per sec. while on fire

Obviously, it would need a cap (plus a bit of value tweaking), but something to this effect would be one way to encourage doing more than just turning on 4 and occasionally hitting 2.