Players who are unhappy with their character's stats could benefit from an NPC that will trade base XP for ATK/DEF XP. Of course there would be a tradeoff, you wouldn't receive 1:1 XP, maybe more like 1:5 or 1:10. So if a player wanted to add 500 DEF XP he/she would have to give up 2500 or 5000 base XP. If there is a concern that it would be too easy to train, then perhaps players could only use the XP they have earned for their current level. Once they have used that up they wouldn't be able to trade anymore until they earned more XP.
I think this would add to the experience of the game and maybe prevent players from quitting prematurely because they are unhappy with their characters and there is nothing they can do about it.
I myself have been trying to raise my ATK/DEF without raising my level for sake of the deathmatch, but it becomes very tedious and somewhat disheartening. I'm not able to enjoy the questing and exploring features of the game because I am stuck training all day.