I guess I'll keep this post short and just throw out an idea. This, like everything else, has pros and cons. However, I think it could be a great addition for all players.
Let's suppose for a moment, that the coliseum was more than just a Gladiators' guild. What if there was an NPC that would offer you a "training" session. For a fee (or not), you could enter an instanced battleground where it was just you and/or your party, an overseer NPC of sorts, and an onslaught of enemies to kill. The enemies could spawn forever as long as you choose to keep fighting. If you are done fighting in the conditions you set, you could speak with the overseer NPC and exit the instance.
There are a lot of things that could be done with this way of leveling up. What if you got training credits that automatically accrued when a certain number of waves was defeated? What if the NPC offered special buffs (bought with the training credits) that lasted only as long as you stayed in the training session? (Fields of Valor style) What if the training credits could be used to buy gear that had special stat boosts that only worked within the training session?
Obviously, you don't want players only using the training sessions to level up on and ignoring all of the open world content. To balance that, perhaps the enemies in the session could not drop any loot/gil aside from training credits. Also, there could be a hard cap on the maximum experience points an enemy will wield. For instance, if someone enters a training session and ends up fighting rank 44-49 hogs; they could receive no more than 500xp per hog. These hogs could be just slightly easier than the open world hogs but you still won't get any more than 500 a pop at best. Whereas, the open world hogs can give upwards of 1k+ with the proper chain bonus. Another idea is to prevent any spiritbond from being built up while inside the training session as well.
This could eliminate some of the claim wars that happen when people are trying to level up. This could also be a means to allow players and parties of ALL setups to adjust the battle to match their form of play beforehand. By allowing players to select the difficulty, the maximum number of enemies on the battlefield at any one time, and/or the type of enemies they would like to fight, they would never truly have to worry about if they would be able to get a good group together for leveling purposes.
Leveling up should never be TOO easy, but let's face it. Some people are just not going to have the friends/time/resources to always be able to level up when they finally get the chance to play. I, personally, would enjoy always having an option for decent experience points no matter what my class or play time is. Like I said, this is just an idea I had. Feel free to add or subtract from it. :)