There is a lot of free games, and a lot of authors of these games are accept contributions. Of course, I never make contributions to free games because of my greed. But, if gamer liked some game, he, besides desire to support the author, can get desire to spread the game between other gamers. He may desire to recommend the game. He can recommend the game to his friends and on forums. But that's free. He can recommend the game he even didn't like to troll other people.
There is no ads on itch.io. There are featured games and selections of games on main page, but that's it. What if there was section on itch.io dedicated fully to ads of free games. Infinite scroll of flashy banners of different shapes and size. When player decide to make a small tip to a free game, in addition he get ability to make this game appear in this section. Actually he get a store of banners. He can choose, does he paying for clicks or for shows, to which users this banner shows (maybe user must like some tag or played concrete game), size and shape of banner, will users see exactly his review of this game or not, will banner have default design or, maybe, he decide to make his own design.