Both defender prestige lines are rather expensive a little clumsy to acquire in terms of their feat and enhancement costs, and outside of their stance benefits their upgrades aren't great (this is particularly true for paladins, who don't get 1 ac per tier like fighters do). Recent reductions to the costs of some enhancements to ease PRE requirements haven't helped because these lines tend not to require Improved Skill enhancements, so they're a bit more expensive and the return on them isn't so good.
The main attraction of tier 3 of both lines is the tower set bonus for each. Outside of this, tanks are generally gaining more benefit from more levels of another class (particularly fighters).
To this end, I propose the addition of an ability that commonly appears on defensive- and protection-oriented 3.5 Prestige classes: Mettle.
Mettle is modelled on evasion and, while it's probably more powerful than evasion in pen and paper D&D, it would generally be less strong on DDO.
Mettle negates the effects of any fortitude or will-based spell with a fort: partial or will: x saving throw (the same way evasion negates all damage on reflex: half saving throw). For example, it would negate the damage dealt by making a fortitude save against finger of death, or remove the shaken effect gained after saving against Fear. This would be a more potent ability with the changes coming to instant death effects, but it still wouldn't be that powerful: players have access to immunity abilities for almost all of these, and fortitude-based and will-based direct damage spells are quite rare (ice lance, disintegrate, etc, all of which do very low damage on a successful save already). This would give defender types an edge in the kinds of situations where they tend to be the weakest - where they may have already died and are being called on to quickly recover a situation and probably aren't fully buffed. The extra protection isn't that significant in any particular situation, but it'd add another level of defense to these classes against non-damaging effects that they presently lack, as they don't gain any non-class benefits against anything but physical attacks.
This wouldn't be the broadest protection out there, but it'd be situationally useful, and its the logical place to use an ability of that nature.