If I could design an MMO-/ARPG it would work something like this.
PvE
The characters would only have 5 abilities each BUT each ability would have "versions" that actually would allow for vdifferent mechanics behind each skill (So, like Whirl Wind, but one WW can lifesteal, another can make you move faster ect). The world would be open and for this game, despite the fact that I don't like instancing, it would be heavily instanced. There would be dozens of dungeons of every difficulty level, and large, huge, amounts of gear. There would be levels, but they would not affect mechanics of game play--which means a level 1 could potentially, given the right gear, hit as hard as a level 99 (Or whatever).
However, there would be "experience" like power--as you can augment abilities with experience (rather than putting it into your level.) Think of this as kind of like AA's, except each ability has it's own little "build up" that you can slowly build into--however, every time you swap a skill, you lose this build up. These would be called "Skill levels". So again, taking "Whirl Wind" as an example--You'd select which "version" of WW you want to build up, lets say it's the "life steal" one--and every level of that you gain experience for, you get slightly more lifesteal. If you were building up the version that gives you higher movement speed, then every level might give you a small amount of movement speed.
These would be "capped". However, if you swap them, you lose the levels on each ability. Players WOULD have reasons to swap, as items will also give levels to abilities, and the +skill levels on items, would allow abilities to exceed their inherent caps. So for example, an Axe drops with +1 Whirl Wind Life Steal skill level--you have that skill maxed at 5% Life Steal, the Axe now boosts it to 6%. (There would be overall soft caps from +Item Skill levels ect--so players would have to watch and diversify their items to get the most out of +skill levels).
Character levels would be used strictly as a fame type system, which would allow you to unlock various dungeons--these dungeons would have progressively higher difficulty and better chances at loot (But maybe not better loot overall--However, I imagine level "bosses" would have small chances at unique loot). The caveat, however, would be that higher end dungeons reduce your level upon death. Level 90+ dungeons would take away 2-3 levels--and these levels could take a significant amount of time to regain--if you are knocked below 90, for example, you're then restricted to dungeons that require 80+ or lower.
Also, these "high end" dungeons would not re-spawn until you die and their mechanics would work similar to an "endless" dungeon--so the deeper you go, the harder, but you get more loot and more chances at those "unique" boss drops. And it would just continue on, the developers adding levels. (Levels would be random, bosses would not be. It should be kind of a source of notoriety to kill X or Y boss in the high end, 99 dungeon--because remember, a death locks you out of for a few days.)
Middle tier dungeons (Lets say 60-70+) would hardly take away anything and would have a far lower hit to levels. Essentially, these dungeons would be experience grinding grounds for when you're locked out of the higher dungeons for dying. These dungeons would still drop good loot, ect, just not at the samequantityas the max level dungeons.
Between tweaking skills to adapt your build to new items. And recovering from progression in the max level dungeons--the game would emphasize grinding for both loot and a constant stream of experience in the various dungeons. And all of this would be to help in PvP.
PvP
The same characters you've been farming on would be placed in a MOBA. When the game starts, however, all of their items are grayed out, and all of their skill "levels" are set to 0. So at the start of the game, everyone is a generic version of character X or Y and somewhat balanced. As the game progresses however, and people farm up gold, instead of an item shop, there would be an "unlocking" store. Each item slot would cost X gold to unlock. So chest piece? 2k gold. Weapon? 2.5k gold. Also, each skill "level" for your skills would cost a small amount of gold to unlock.
Unless you become extremely fed in the game, you won't unlock all of your gear or "skill levels". It's up to the player to decide which pieces of gear or skill levels will be the most beneficial to him in this game. As the game progresses, and people's wildly different gear comes into play, obviously things will become less balanced--a very farmed character will be a good deal stronger than one who hasn't farmed a lot in the PvE game.
However, the start of the game, everyone is even--so a better player will unlock his gear and "skill levels" far faster. Allowing him to potentially end the game with skill before the better farmer/more geared character can really access his power. So the first half of the match, the "lane" phase, would be determined mostly by skill--and even a better geared character can be shut out if he's outmatched. But the second half of the game, all that farming and character build up would come into play, so if two people are relatively even in skill levels, the one who is better farmed will be stronger.
Anyway, just a game I'd design if I could design them--I think combining the need to farm to be stronger WITH an allowance of skill and balance at the start of the match, would make a neat game. Champions could literally train out in the world and become stronger for "the arena", but player skill would have a bearing on how they can use or be locked out of that strength. I'd find it addicting, but who knows.