Keep in mind that it's not likely that many people would be on at the same time. It would be better to focus the environment on supporting interactive RP and solo adventuring, plus maybe some PvP arena areas, if for no other reason than that models existing gaming patterns.
Combat types are typically well-suited for grinding for XP, which is something I'd personally like to avoid. I think the instances should be more interesting than 'go through portal, kill X creatures/units, return'.
Having said that, perhaps we can make a 'magic-kingdom-like' area for barbarian warlords, and allow Hatamancers to summon them into instances? Optionally, perhaps barbarian warlords could just exist within their own instances, and either allow casters to enter them or agree to be summoned.
Hatamancers, Moneymancers, and Thinkamancers would need to be implemented as NPCs, I think, to prevent abuses. Thinkamancers especially should only be NPCs, so that while all casters could learn some few spells in other disciplines, we could avoid the caster link issues by making them plot points for instances rather than letting players experiment. With 24 caster types, we could forbid a majority of them and still allow for multiple play styles. For example:
Permitted: Turnamancy, Dollamancy, Dirtamancy, Dittomancy, Lookamancy, Foolamancy, Flower Power, Shockamancy, Croakamancy, Healomancy
Forbidden (Not possible or NPC only): Retconjuration (obviously), Hat Magic, Predictamancy, Thinkamancy, Moneymancy, Changemancy, Mathamancy, Findamancy, Luckamancy
Need more definition: Weirdomancy, Signamancy, Date-a-mancy, Carnymancy, Rhyme-o-mancy