Here is how I would go about class design were I in their shoes:
Design a whole bunch of abilities that can function within the voxel dominated world you have invented and which fall in line with the overall basic concepts of EQ. Go crazy with concepts and implement anything you can possibly imagine but dont worry about number values or necessarily who gets what ability. This represents the metaphysics of movement, combat, and environment interaction.
Put very neutral values in for the necessary ability parameters and set your designers / QA staff / whoever loose to use whatever combination of 8 abilities they want and tell them to go crazy and monitor the results.
Why?
A sufficiently large and creative group of people actively trying to break the game will almost certainly be able to do so far faster than any particular math model you can cobble together that tries to incorporate all the different variables of how you move and interact within this new destructible world. No sense in putting an inordinately large amount of effort into developing 'classes' and abilities until you get some empirical results on how they really function.
Armed with what it looks like people do when they actually have access to everything you can then start to make choices about how likely it is to have access to everything and if you want to ever allow them to slot whatever they want from whatever pool of abilities. You can then form the basis of your 'core' classes with the most fundamental sorts of abilities and see how the game plays when they have access to more restricted lists of abilities that correspond to the various archetypes.
Really fundamental questions need to be decided by the team right off as they will make huge differences in how the game plays. Do tanks make use of collision detection? Do you intend wall / persistent effect creation plus collision detection and the fully destructible environment to play a significant role in establishing how the combat flows? Do you want to reward specialized roles with increased effectiveness in doing a particular function (tank, dps, healing, cc, support) or do you want to allow nearly unrestricted synergy?
Once you know these things then you can really start to solidify the classes and make them really feel like Everquest, but it does you no good to focus on creating a whole series of Enchanter base and specialized classes, for example, if mezzing and charming are no where near as efficient as just blowing the crap out of everything and healing through every encounter.
Whatever is done and however it is done I thoroughly believe in two core concepts:
1) Cure is bullshit. Every debilitating effect in the game that has to be there for important encounter reasons is always tagged uncurable and every other DoT / debuff ends up being a can-timmy-hit-the-big-red-button-fast-enough mechanic with various different flavors of cooldowns and effect types and other such things. Balance your game around there being no such thing as an instant Cure and you will be much happier in the long run.**
2) Healing is the very last thing you work on. Far too many games have gotten fucked up from day 1 because someone had a really cool concept for a healer (Complete Heal in EQ, Chloromancer in Rift etc.) that then required all content to be designed with that in mind. Allow all movement, avoidance, mitigation, crowd control and such to mature and then put in healing that compliments and enhances the experience rather than dominates it from day one.
If done right the physics of this new creation could make for very very different combat concepts that compliment synergies and looks much more tactical. If done badly then we'll end up with carbon copy flavor-of-the-month characters all standing under the dragon and backstabbing him in the balls while the beast stupidly swings at the most heavily protected person in the room who also happens to be bathed in a deluge of healing magic.
**For shits and giggles you can make some debilitates the result of persistent magical effects that, because they are made of transparent voxels you can move through, are destroyable and thus "cured" by damaging them with the right spell (Dispel Magic) and which are immune to all other forms of damage.