I can see applications in multiplayer gamedev - imagine being able to run the whole game simulation on a clients machine and have them assert back to you that they killed 7 goblins, looted a rare sword from a chest, and died 3 times - and you could just trust them.
Your server costs would only need to be for the metaprogression/persistence related stuff that could be done relatively infrequently based on updates from the client.
I agree. Exploring this in game worlds came up in a job interview a few years ago :)
ZK proofs are potentially a transformative tool for real-tine distributed systems in general, not just games. They potentially improve laency ("ping"), by changing the communication patterns in a distributed consensus system. That's great for games and other real-time systems.
It would be amazing to have a single authoritative server in an mmo for the EU, USA, Asia, Oceania etc regions and only have bad latency if you were actually directly interacting with people from far away.
Your server costs would only need to be for the metaprogression/persistence related stuff that could be done relatively infrequently based on updates from the client.