Currently In PvE:
Magicka Nightblade, Magicka Sorcerer, and Magicka Templar are all pretty close/competitive in terms of dps.

Magicka Dragonknight and Magicka Warden trail pretty far behind.
When was the last time you saw a Magicka Dragonknight in a competetive raid environment?

There is one easy skill change that I'd suggest to remedy that, and that is to Molten Armaments.

Currently:
My Molten Armaments IV tooltip reads:
Charge you and your allies' weapons with volcanic power to gain Major Sorcery, increasing your Spell Damage by 20% for 33 seconds.
Your own damage with fully-charged Heavy Attacks is increased by 40% while active.
(without Eternal Mountain passive, which increases duration by 20%)

Proposed Change:
Molten Armaments IV:
Charge you and your allie's weapons with volcanic power to gain Major Sorcery, increasing your Spell Damage by 20% for 20 seconds
While this ability is active you gain Empower, increasing the damage of your Light Attacks by 40%.
(without Eternal Mountain passive, which increases duration by 20%)

Conclusion:
DPS in mid to endgame PvE don't heavy attack often. This is because it is a dps loss to heavy attack over a light attack + spamable. Even with Molten Armaments active, heavy attacking is still a dps loss. This small change to Molten Armaments would instantly increase PvE Magicka Dragonknight dps by a fair bit, hopefully enough to make it at least competetive with the other three top magicka classes. I'm still not convinced many people would play Magicka Dragonknight dps... because even with this change I think it would still lag a bit behind the other classes. But it would be a small gap (we're probably talking 2-3k dps less rather than 7-8k) and at least allow them to be viable as a dps class.