So much great replies from everyone.
I spent some time pondering levelless MMOs a few years ago; I don't actually consider the question important any more because I realized I prefer MMOs which have no PvP, or which have a PvP system which has different combat than the game's main PvE combat. For example, PvP might be a CCG system while the main combat is action or spellbar/cooldown. Or PvP and PvE might both be based on using bred pets as units, but PvP would have 'preconstructed decks' of pets that new players could use if they wanted to go right into PvP without playing the PvE portions of the game to create their own pets.
This is a good idea that is proven to work, except in RPG games.
Where giving players starter packs in CCG games allows them a good starting point. In time thy can add to it make it there own and still have a deck that could standup to good players if thy are skilled with the game, means that thy all stand on the same level and it's there own skill that determines where thy stand.
Giving players a starting party in a RPG makes the player feel like you are forcing them in one direction, allowing them to add new characters means those characters start at a low level or is a pre-made character. Also fighting players who have higher level characters means that thy would always stand above you.
Now you could level the playing field by lowering the highest level player down to the lowest players level. however the player who was at the higher level has a party who is made to use the advantages of higher levels, and now thy will either lose some attacks or those attacks are replaced with weaker ones that thy are not familiar with.
In reverse rising a player to a higher level means that there party will not take full advantage of the new level, like the other players party.
Then there is the score or rank system, now players with similar score or rank can battle each other but not those higher or lower.
This system is liked by newcomers, it gives them time to get used to the game without being bullied by players and game mechanics. however it divides your players from each other.
The worst thing you can do in a MMO is prevent players from interacting with each other, its the whole reason thy play MMOs.
If you allow for some option where advance players can challenge higher scored players, you will have to rise or lowers there levels. a Level 20 can't beat a level 100.
You could always add social thing and other ways for players to interact, but it means most players will then avoid battles as it is one of the weaker features:
I don't actually consider the question important any more because I realized I prefer MMOs which have no PvP
Did you check Destiny mechanics?
Yes, I have. I feel that FPS games don't depend on stats like RPG and so you could easily getaway with things like removing levels and stats, mostly because it's the players skill that determine who wins not there level.
Shooters that heavily rely on levels like Borderlands and Mass effect, is grouped under RPG for this dissection.
Removing Xp systems isn't always a good thing in FPS multiplayer games, I would hate it if thy removed Xp from Call of Duty games. The Xp system in these games are a lot different in these type of games than in RPGs.
I've always wondered why MMOs didn't adopt an open ended leveling mechanic where there was no level cap. However...XP required for the next level would increase exponentially. Levels would be harder and harder to attain so you'd need more and more xp to gain the next level and not just a fixed amount.
I think the reason this is avoided in MMOs is because it fails even in single player RPGs, eventually the player reaches a point where there is no point in leveling up and monsters die in one hit.
You could level the monsters with the player, but it gets boring fighting the same thing over and over, even if there color changes, so you fill the need for challenge but not content.
I take it back...there WOULD be a cap in this type of system: time. But instead of artificially restricting progress you're leaving it up to the player how much time they want to invest to level up. As a developer I don't this would pose MUCH change in the way you design "end game" challenges. You'd have to consider what the mean player level would probably be, and what the max level on average would probably be.
You are right, there is a cap. This can best be seen in games that have a rank system like CCGs, there is always some score level that no one has passed.
A Give-Take exp system in which getting exp somewhere lowers it somewhere else. There might be an underlying 'base' level that improves but perhaps they are few and far between or sport a more fluid development paradigm, but improving certain stats or skills would force the player to underprioritize the other. Actually 'grounded' in modern MMO design because improvement is more predictable (the player has a 'goal' to work towards).
I have been thinking about something like this, where training a player in a stat like strength costs a stat of speed.
This would be a Tier system, where a Tier 1 character has 10 stats to move around and a Tier 5(highest) has 100 stats.
So if a Tier 1 fights a Tier 5 thy multiply by there stats by 10, now attacks should be affected by stats and have no stats of there own.
This is similar to Pokémon games, except thy use levels not tiers although evolution can be seen as tiers. Thy overcome the power difference in evolution by allowing a lower evolution to learn new skills faster, but even so high level players will always have a better chance of wining.
A Use system in which skills are always in flux
Never should you make a game where players lose power over time, unless it is a casual game.
Fallout 3 is a good example. Each time you fired a gun it would lose some of it's condition and a percent of its stats.
This meant that you would never do the same amount of damage to a enemy as in the first shot.
In Fallout new vegas, thy fixed this with a extra bit where the weapon was always at a 100% condition
Class-Related Design? Perhaps certain classes could explore regions whereas not others. For example, I need an ice mage in my party in order to do X- so we'll get one for that purpose. Marry classes to regional progress. Perhaps have some classes 'thrive' in an area but the potential rewards aren't quite as exciting. Take them out of their element and put them somewhere else where they may not be as 'useful' but when handy they really help. Just an idea.
Tiered Map System: Perhaps you could have two intersecting world map ideas: For example, imagine having an underworld and an overworld in a game (Terranigma?) Then throw in varied levels or difficulties into either. For example, in one difficult region on the overworld you'd have an easier one below-grounds and vice versa. This is a very basic concept that would require intelligent designing in order not to be utterly stupid. I mention it just out of curiosity. I certainly couldn't make something feasible out of it. Perhaps you could have characters that specialize in one 'domain' over the other or something.
This again divides players from each other.
Challenges that need X type or you can only enter if X, do add to the game if the player can change to X if thy want or if there are alternative ways to do those missions when you don't have X.
Players who play games with leveling systems enjoy progress and solving problems, this means there should never really be only one definitive answer to a problem for them.
Final fantasy 13 as a example, has a set of attacks that work best on each monster. This meant that most of the time the attacks the player chose was the same as what the auto battle chose.
This meant that the game didn't appeal to players who viewed each battle as a problem that thy needed to solve, like having puzzles where the only way of solving them was to think like the creator of that puzzle.
Thanks for all your great replies, I still have a lot that I would like to write about.
I think that the reason CCG, MOBA and Monster Breading games do so well in MMOs is because thy where made for PvP, but why do RTS games fail at MMO?
RTS are made with players battling each other in mind, yet thy are one of the most unsuccessful genres at MMOs.
edit: Spelling and other, I am not native English.