definitely a little bit of shameless self promotion but I write about a lot of common harness concepts in the docs of Wingman (https://wingman.actor) - a client agnostic agent harness I've been working on for a while:

https://docs.wingman.actor/concepts/sessions/

I think there is still plenty of innovation to be had in the space. Right now everything is still pretty bundled to the file system where getting skills and things to work inside of like a virtual worker requires simulating a virtual file system (usually). I believe even in the latest opencode v2 versions it still requires that a session be bundled to a working directory, I try to avoid (some of) that in Wingman.