DoT Mage

[Seraphor]

They could easily solve the debuff limit by giving SMN/SCH a single DoT that you bolster with their own buffs. So instead of it being multiple DoTs and multiple debuffs, you stack personal buffs that increase the potency of your DoT.
Cast Bio to inflict a 40 potency DoT.
Stack a Miasma buff to turn that Bio DoT into an 80 potency DoT, etc.

The problem with DoT mages though is having the rest of their expanded kit having any kind of substantial impact.
Either the DoTs are so weak as to be inconsequential or all of your combined DoTs account for most of your damage output and your big showstoppers (like Bahamut/Phoenix, etc.) aren't very impactful.
I think combining DoTs with Summons (which acts like DoTs themselves anyway) was always a bad move tbh. It's really stifled the jobs growth potential.