I tend to believe any game must first be a single player game to be fun and then a connection to other players that allows interaction. This means any game system must first be playable alone. For example: Cooking, where you can place something into a stove, like bread and have it cook for 8 hours and then come back and cool it, to be sold. To make this a single player, you would need to be able to sell your bread to an NPC at the Bakery. To make it fun, you need to set goals, like having 60 Cooking, you can bake bread and at GM Level, you can make Apple Pie. Also, you have different cook times and only allow so many stoves at GM Level. Then you can make GP as you master dishes. Say, you cook something 100 times and a new dish comes on the menu, like pizza and as the pizza is better made, it gives a better bonus if eaten. So, now we have branched out into a multi-player where we can sell to other players and they have a need for the item, perhaps, a +5 in skill, when eaten. We also see a connection to wheat or apple trees. So, we can have farmers that can buy a field and grow crops, who can then either sell to NPCs or Cooks, whom also need to master seeds and skill to be able to grow them. So, again we have gone from a single player to a community of workers. This is the point in the game where you can no longer do it all. You will have Farmers and Cooks who spend most of their time doing only that, mastering. Each could give better abilities where a cook could use better grown crops and sell to NPCs for more with a better bonus for whomever eats the food. So, craft skills would become a type of mastery as well as base skill to allow for better resources, crafts that are not available such as Claw Foot Table. Also, types of fish for example, rather than just skill. You would catch a few fish and as you master those others would unlock, so as to enable it to take years to master all of the types of fish. This brings us to Arenas, where they are first trying to be multi-players rather than single player first. So, in theory, they should be PvM'd based so as you kill types of spawn you get to fight tougher spawns or mastery. This would make you the ultimate spawner if you could solo the toughest. So, now the multi-player part where the ranking low fighters are spawned, where a copy of their character is produced and you can work your way all the way to the best PvP guy and beat his spawn. This would cause the real player to challenge at a certain level of reward, such as, 5 mill, 10 mill, and 20 mill. So, as in the single player you are ranked and also receive GP to make an honest living but can also go live and win big along with some side bets. You could even include a new player track that would slowly feed the new player equipment, such as the first spawn mastery could give a weapon of choice and work the new player up into a basic suit and skill range of combat.