As we should all know, a huge successor of this type of free-to-play is League of Legends, created by Riot. Their only in-game purchases you are forced to spend money on is skins, which only change the aesthetics of the game. However, because I appreciate all the work and effort that is being put in this game; I spend plenty of my money on skins to keep their money going.
In effect, it's not about what you're giving your audience in terms of bought items; but rather putting them in a position where they WANT to support you. There are various ways to do this but, in reality, the simplest one is to put the player first.