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When you log in, you custom-create a character using a numerical pool for stats you distribute as you wish according to a weighted system. You pick from available basic gear that has X amount of stats. These stats never increase, but they can have different distributions . You also pick your gender, race, name and can invent a name for your own profession.
Next, you pick from a base assortment of skills, talents, traits, etc. These are like cards you collect, but everyone starts out with a base selection. You fill out your skill/talent/trait slots as you see fit.
Skills, talents and traits are based off of the non-comparable paradigm found in League of Legends. You can collect a vast assortment of different skills, talents and traits and create different builds when you start up the game client, but each time you log into the actual game world all character specs are locked (this doesn't extend to gear, food, potions, etc. - just stats, traits, talents and skills). All trait, talent and skill "cards" you collect are usable by any character you create.
Thus, the entire game world is essentially flat. There is no need for "starter" or "midrange" areas (that soon become depopulated). Right outside the log-in area, or on the other side of the world, it's the same "level" of PvE. The world has dynamic events and quests, vast solo areas and instances, raid areas, dungeons, WvW, PvP arenas, dueling, group-oriented boss mobs, etc. There can be different areas that have different crafting components/resources - note: there would be no need for different "levels" of crafting. All recipes would be worthwhile, depending on your build.
There can be many ways to collect gold or various tokens for acquiring new trait, skill and talent "cards", as well as various kinds of skins for your character and for gear. The cash shop can sell skins and lock boxes with random cards amont other things. It might also sell certain standardized "card" sets or animation/sound effect packages, bank slots, character slots, etc.
Expansions can drop in entirely different horizontal concepts - politics, exploration, fighting skills, crafting, etc. - with appropriate "card" sets, gear stat arrangements (not increases), and skins. Modules can be offered (for a price) where players can design their own gear styles, furniture for their homes, guild or other appropriate customizables.
You wouldn't even have to create a huge world to begin with. League of Legends started out very small. Start-up costs would be much lower than the usual MMOG because you could just cut out 90% of the stuff other MMOG's are full of that nobody uses more than a few weeks anyway to get to the end game.
In a purely horizontal MMOG, the whole world is in fact the "end game", so you need none of that other pre-end-game content at launch. Since it is only going to expand horizontally, then whatever tokens or gold players amass by playing before expansions come out will be usable for the new horizontal content, giving them reason to play in-between expansions even if they amass all the "cards" currently available.
Such an MMOG could offer enormous long-term horizontal opportunities - to build houses, estates, even cities; become famous for many different reasons; explore factions and pursue political prestige, become an explorer, build boats or airships ... the sandbox-style possibilities would be endless. And, in a flat game, there is no "pay to win" as far as a cash shop is concerned, so the game could be monetized for high profit. The role-playing potential would be enornous.