Item idea:I stumbled across this thread and instantly thought. Rare candy.
Item Effects:It's obvious that it would increase a certain McMMO level, but the item itself would have to be difficult or impossible to obtain, as to tone down the amount of powerlevelling it can cause. The items themselves do not even have to be an entire level, they could be a set amount of exp per item.
Item ID:The item I had in mind at first were Ghast Tears, but then again only certain people train on ghasts. Diamonds? Although some people can just pull armour out from anywhere whenever they like, But this could drive the price up and make them more wanted and stop people buying ridiculous amounts of the material, also it makes the player decide whether to use the diamond on levelling or gear.
This is my suggestion and I hope you approve. (Please don't reply with some 1ce'y sarcasm).