Flame Lash
Because Flame Lash cannot be stored out of combat for instant ganks like Relentless Focus, it is fine for it to remain strong. However, the current AoE burst is too forgiving and doesn't let you roll dodge out of it.

1.Make Flame Lash single-target and give it a 4 second DoT that extends the duration if it's already applied.
2. Make fully stacked Molten Whip the AoE morph for players who want to maximize AoE synergy because you can't spam it on people who are roll dodging with it since it only hits once every rotation.

Heart of Flame/Soul of Flame
Heart of flame lets you get all sustain from one ability that cannot be interrupted, can be used while blocking to permablock, does not cost health, heals you, pays for it's magicka cost, and can be used as part of a burst combo.

At maximum value, it can restore up to 45% of both resources every 5 seconds. If a player has around 40k total resources, the recovery value is extremely high. Even after accounting for Major Recovery buffs and comparing it to Mundus Stone recovery/power values, the sustain can be worth thousands of weapon and spell damage worth of recovery. Even in a more realistic situation where the player casts it around half resources, it can still be worth over well over 2,000 weapon/spell damage worth of sustain.

A good nerf would be to reduce the resource return by at least one-third while the player is not immune to disabling effects. This makes it less effective outside of combat and would create counterplay: a person or coordinated team could avoid CCing the target for about 30 seconds until they start having resource issues from relying too heavily on one ability for all their sustain.