All hail the Tank Mage
The concept of the Tank Mage is as old as Ultima Online. For those not familiar with the term, check this article on gamasutra (it’s an interesting article about hybrids in which the Tank Mage is explained on the first page). Basically, the Tank Mage has the survivability of a tank and the power of a mage. What a surprise. If the mage has healing spells as well, the Tank Mage is a holy trinity all by itself. Designers try to avoid Tank Mages for PvE because it makes other players redundant and for PvP because it is by definition more powerful than any other concept.
Tank Mages in an MMO with classes are bad. They can solo more than anyone else and if they have to group they can stick well under the intended group size. It’s even worse in a PvP game. The constantly shifting of class balance can suddenly allow one class to become a Tank Mage and dominate, while the other classes can do nothing about it. Therefore, Tank Mages should be avoided by the designers to keep their game healthy.
Tank Mages in an MMO without classes are unavoidable. If players have completely free choice then they will gravitate to playing the sturdiest character with the best damage output. The question is if this is a bad thing. In terms of PvP balance it seems no problem: if everyone is overpowered, then no one is. However, this is only true if damage output and survivability are balanced. Let’s use Champions Online as the example. While Mindful Reinforcement (ranks 2,3) was bugged, people had incredible survivability, but not enough damage output to defeat each other, which led to very long or infinite battles. On the other hand the current situation sees very high damage as done by STC (for rants about that, check the forums, in short: it removes 55% of someone’s health in 10 seconds while ignoring all mitigation) and (de)buff stacking with force cascade and power armor which leads to oneshots. In the first situation the Tank Mages can kill anyone but other Tank Mages, while in the second everyone can be killed, it just depends on who gets the first shot off. Neither situation is desirable.
PvE balance is just as difficult. Can the designers assume everyone is a Tank Mage? Probably not, which means some teams will struggle while others will breeze through the content purely based on power selection without any regard to individual skill in playing. Also, if everyone can do everything then the roles (tank, healer, damage) players can take up are gone as well. Nothing makes a WoW dungeon fresher than going through it in another role. The lack of roles will actually cause burnout sooner.
The worst situation that can occur is if it is very difficult to make a Tank Mage; only one or two builds qualify as one. Because all information about builds is open, many people will jump on the flavor-of-the-month turning the beautiful open choice system in practice in a one class game. This has already happened in CO to some extent: powerful PvP builds have scared many people away in tier4, while forcing everyone to pick up STC in lower tiers or die.
Since an open skill system cannot avoid the Tank Mage, it should be relatively easy to build one and there should be many viable paths. This will lead to a spread in skills and choices that will make the overall game easier to balance, while individual players keep a sense of power and versatility (especially while soloing). Also, because of the many paths to the Tank Mage, there will be some role diversification anyone, e.g. some will have more tank than mage and vice versa.
So, while the Tank Mage is something that should be avoided in class-based MMO’s, it is a concept that classless MMO’s will have to build around to provide a fun and equally challenging game to everyone.