Melee is not hard to balance, but many MMOs struggle with it. Why they struggle is very simple and has nothing to do with DPS v Mitigation v Agro and everything to do with Developer mindset.
All MMO developer teams have two gauges for what makes up a tank: how much DPS it can do before they think of it as a DPS and how much damage it can take before they think of it as a tank. Permitting anything to cross those lines is abhorrent -- they'll often consider crazy changes to keep those lines "safe".
In DDO, the DPS gauge for tanks is lower than any other MMO I've ever played. It's basically zero. If a tank in our raids gets a kill, he makes fun of the entire raid.
The mitigation gauge is similarly pegged in DDO. If you're living through two hits as DPS, you're stealing the tank's show here.
In most MMO's, a tank does about 60~80% of the DPS of a DPS-focused character. They usually solo amazingly well, often much better than pure DPS. They are still not played as often because tanks are seen as leaders and most players just want to have fun without responsibility for other's fun.
Same thing for DPS playing as a tank. Usually they can, but not at the highest levels and not without extra effort. In DDO, this is almost impossible -- can be done, but it's v.rare and the healing effort required is pretty extreme (eg. spam healing).
So, you want to balance melee in DDO? The first step is to adjust those dials. You have them pegged at extremes that are making balance impossible. I think you'll find that trinity play will GREATLY increase when tanks can do enough dps to solo simply because that's the major reason I hear for people not liking to play them in DDO.
BTW, adjusting those little cultural bias dials on tank v DPS will not be easy. The only MMOs I've seen make even a slight adaptation there was FFXI (for blink tanks) and EQ1 (for monk FD pulling & tanking).