Simple changes to Warrior-Mage that can make it worth playing.
Enable use of 2h weapons and shields. Walling them off without reason is pointless.
Argument: But Warrior-Mages must always have a free hand to cast magic.
Counter: Can still use shield with one hand and free hand uses magic.
Argument: But two-handed weapons don't permit use of magic.
Counter: Using magic or weapons in combat is player choice. Always has been. As is the choice to drop their weapon in favor of using magic at any point.
Argument: But Warrior-Mages are jacks of all trades, not masters of them.
Counter: Hence restriction to C - class equipment and mithril armor, which prevents use of the highest Defense armor and most damaging weapons.
Enable use of Arcana spells up to Magex5 level as appropriate for every level of Warrior-Mage taken, up to Warrior-Magex5.
Argument: But WaM's are not masters of magic, so shouldn't have access to higher spells.
Counter: And they don't. Magex7 is the highest, and would compose mastery of Arcana spells. Magex6-7 spells would still be walled off, preventing the Warrior-Mage from ever mastering them.
Argument: Then why not just keep taking Mage at each prompt instead of upgrading Warrior-Mage?
Counter: Result is still the same. To gain Warrior-Mage and then upgrade Mage would still only result in Mage upgrading to Magex5 at best. So, regardless, Magex5 spells would be the most they are capable of, even using this method.
Argument: Then why not just do that?
Counter: It'd be the same as punishing the player for even choosing the Warrior-Mage class, and would make the classes very existence irrelevant to the game. If that were to be the case, then the class itself shouldn't even exist.
Adjust the "Cosmic Balance" disadvantage to stat equality between two primary combat stats, not all four.
Argument: But they're not masters of magic or combat. Why lessen the restriction?
Counter: Their lack of mastery is already reflected in their inability to use the best armor, weapons and spells. Mastery or lack-there-of has nothing to do with combat stats.
Argument: Then why bother restricting to just balance of two stats?
Counter: Ultimately that is all that is required for any class to be effective in offense or defense. Any character, regardless of class, can simply pool stats into Str and Res, or Int and Agi, and have a high degree of attack or def without having to focus on just those necessary to their class.
Example: Pooling in Str+Int would be beneficial for ranged physical and close magical attacks, giving high offensive ability, but only moderate ability in CloPhyAtk/def, CloMagDef, and RanMagAtk/Def. Doing in the other way, pooling into Agi+Res would have the opposite effect.
This ultimately reflects how each individual player chooses to act in battle, regardless of class choice. The only difference here is that Warrior-Mage has to keep two at around the same level, whereas any other class can simply pool everything into a single stat.