I think that the Trainer is just as much a focus as the Pokemon that battles with/for them. Otherwise, all the Pokemon games would be like the Mystery Dungeon series. This is reflected in how the Trainer Card "evolves" as you complete different achievements from Gen 3 on. For example, your Card gains a star and changes color when you beat the Elite Four, and again after completing the Pokedex, so on and so forth.
In PokeMMO, I don't think that there really needs to be any leveling for the Trainer. If the Trainer Card still changes color and such, and you can view a player's Card before dueling them, I'd say that's good enough.