A system similar to what Steam has: Trading Cards that can be unlocked by engaging with the streamer. (Requires the streamer to have such feature enabled; The streamer can design and upload their cards and set conditions such as drop rate, how to get them, say by following, subscribing, donating etc.)
Each user would have their own trading card menu in their personal dashboard, to check which have been unlocked, from which streamers, etc.
Viewers can also trade cards with others if they have doubles, etc.
Collecting all cards from a streamer could also unlock a badge or whatever the streamer decides the compensation is (say access to all channel emotes without the need to subscribe, for example).