I basically will have collectibles with a XP system but think it will be best to create a hybrid decentralized / centralized approach depending on the nature of the game's computing tasks. For example, the tracking of XP grinding between player levels will be tracked through a centralized server infrastructure and, once the threshold is reached (new level reached), the information will be committed to the blockchain along with upgraded stats of said character. This will reduce the frequency of transactions and minimize overloading the network.