I love classes and class based systems when they are full of unique and complex classes. If runescape had classes what would they be?
Base this off of skills, costumes and other dress, NPCs and history. Also with classes based on costumes think of what skills they would have. I'll start.
Mime
You have a Mime costume so of course a mime class.
A mime would have a mime skill that acted like a mixture of prayers and spells but drain his run stat. A list of possible actions and their effects. For example, miming climbing a rope would exit him out of a dungeon back to the entrance. It would only be usable once a day and cost 100 run.
And the glass box would grant him invulnerability until he runs out of run, but he couldn't do anything else during his state of invulnerability so its something only useful for group questing.
Necromancer
Skull sceptre and shade robes, plus all these halloween items. Its a given.
Necromancer skill would act like summoning, though more combat oriented. A necromancer could summon zombies and other undead, perhaps as many as 3 at once and they would have multi everywhere. However they would drain prayer and eventually disappear.
Commando
These would pretty much be runescape's answer to the bare fisted monk. They wear the fatigues and get a fisticuffs and ambush skill and only real weapon they use would be daggers for their ambush. With ambush the commando becomes invisible and simply waits unmoving until a target walks by. Commando can also set traps like pitfalls.