There's something fundamentally wrong with the base damage scaling differences that can't be solved without some major rework.
The scaling sources *almost* equate to one another, and yet, when it comes to single target damage, and even 3-target damage when it comes to 2hf, spellcasters fall behind. Fast.
Spellcasters are supposed to trade higher gearing restrictions for greater burst potential.
Which, to be fair, they currently at least partially do, in that the first second of combat (without the Reaper penalty), a spellcaster casts their strongest spell, and, for that singular second, their DPS is a decent notch above the nearby fighter's.
The problem is, no single fight outside of low-skull heroics is short enough for that burst to be a meaningful part of the fight.
As soon as a fight lasts longer than a full caster's rotation, their DPS is already somewhere in the gutter - they already had to resort to worse and worse spells as they waited for the better ones to come back.
Now, we can't exactly make the fights that much shorter everywhere either, that'd just make casters OP.
You COULD always make this a resource race, where casters run out of mana to get there, so they can't do that every fight. But this would be part of the major rework, and it'd be a huge hit to any poor undergeared toon.
The solution would either need to change the base damage everywhere, change how metas affect spells so their benefits don't become a smaller part of a caster's damage as they get better, or give casters something that ramps their damage as the fight drags on, so the DPS on the first rotation would start where it currently is, but end up a bit higher, favoring the longer fights DDO seems to throw at people, at least so long as you have the mana to sustain it.
That last bit SHOULD be the role of stacking DoTs, but the ones we have.. aren't doing it.
Personal suggestion would be terribly long CD spells that automatically reset to a small timer when a fight starts.
So, huge burst DPS spells that work only once per fight, and some time after the fighters have already started doing their thing.
Alternatively, "execution"-style spells, that only do full (extreme levels of) damage on severely damaged enemies.