Caster LB is already quite strong.

Caster LB1 beats melee LB1 on 2 targets.
Caster LB1 beats melee LB2 on 4 targets.
Caster LB1 beats melee LB3 on 7 targets.

That's just talking about the LB1s.

A DoT limit break would just not ever get used. People want to limit break for instant gratification. It's meant to be an epic moment with a powerful immediate effect.