Massively reports that the US version of Runes of Magic will be adding an additional class option to its dual class system in April.
The present and the possibility
Currently, each character has access to two of the game's eight classes (with a further restriction that two classes are exclusive to each race). At any given time, one class is your primary and the other is your secondary, providing access to some of its basic skills and a handful of unique "elite" skills that can, in some cases, fundamentally change how the class plays. For example, my Druid/Rogue learned a primary nuke spell powered by the Rogue's infinitely regenerating energy bar, rather than the druid's standard mana bar, leaving the entire blue bar for self-healing.
Massively's post repeats a rumor that the new system will actually give the player full or partial use of three classes at once, but the comments suggest a less balance-altering suggestion - apparently some international versions of the game currently allow characters to have ACCESS to three classes, while only permitting the USE of one primary and one secondary class at a time. (This would explain the odd class-swapping UI - there's no reason why you'd ever want to do without a secondary class once you've earned the ability to use one, but the game makes you select your one and only second class from a pulldown menu when you switch them.)
The three classes each have their own exp level, so players will have to obtain the exp to level the third class from one to the cap if they want to use it. The devs are probably figuring on selling more exp potions this way, since that is one of their primary sources of revenue. Storage could also be an issue for your third set of gear. On the plus side, you could keep the third class separate for running low level group content, or playing with a static group.
(The most efficient way to level the third class will probably involve using your existing high level classes to grind out some daily quests, and then switching over to your new low level class to turn them in for large exp awards - I was already running low on content in the 30/30 range with two classes, much less three.)
In terms of options, players will be free to take the "optimal" secondary for their preferred role in group content and an additional class for other purposes (e.g. solo or PVP). Alternately, a player could fit all three holy trinity archetypes on a single character (again, with a potential issue with gear storage). This also insulates you a bit against future nerfs - with six possible pairings, it's almost impossible for all of them to become unplayable.
Butting heads with Rift?
One commenter suggests that this move is intended to combat Rift's more versatile soul system. I maintain that this change is more about selling more potions, storage, gear, etc than a direct competition. Though both systems will give any one character access to about a quarter of all possible specs, Rift is designed to allow players to swap on the fly to up to four specs. ROM still appears to be designed around less frequent changes.
That said, it will be interesting to see whether the addition of more options leaves one or both games feeling ironically like there is less variety left for future characters, when your first can fill just about any role with an appropriate spec switch.