Well, here's the thing. Let's put the three races compaired to other MMOs.

Humans- Like in most MMOs they're balanced, while having no real weakness they have no strength

Saiyans- Since they're the brutish race of the series they're more of a physical fighter, they'd have good physical attack and defense they compensate with low ki (the DBZ world's version of Magic)

Nameks- Since they're a wise race with long lifespans they would have low physical stats but make up for it with very powerful ki attacks and skills.

Also, there are more sentient lifeforms in the Dragon Ball world than just humans, saiyans, and Nameks. The planet 'earth' that world has is inhabited by humaniod animal-like people (dogs, cats, bears, etc), and what about the other races shown off in the series, like the Icejins (I think that's the name of Frieza's race), or maybe even a race of Androids (like Android 18 *fanboy crush*), also, what about the demon-lime monsters from King Dabura's world, or maybe a hybrid species like Cell *crosses fingers*? Not only would their be all of these races but alliances would be in this too. You could be like a Z-Fighter and protect the galaxy from random badguys, you could be part of the evil Empire that was once led by Frieza's family, you could even be part of the Majin mage clan. That also leaves out another thing, character classes. You can be a fighter that fights along the lines of Master Roshi, or fight the brutelike style the space-alien badguys fought (Ginyu Force, etc) or probably use nothing but magic/ki (the nameks, the Majins, yadda yadda).

*puts away DBZ nerd hat*