It's great to see that we are starting to think about serialisation again. After gaining the experience with Codable over the years and understanding it's strengths and shortfalls I definitely think this is a topic worth visiting.
This may have been mentioned already, but something I would love to see is built in lossy deserialisation of lists with failure handling for logging. People have built extensions for this in Codable but having something built in would be great to see. This is valuable for lists of data which you know is going to change over time but you don't know how it's going to change as those decisions will be made in the future.
I've also had to fall back to using JSONSerialisation a few times over the years as it gives much greater flexibility to deal with payloads which you may not necessarily know the structure of up front, or where you may want to dig deep in to a JSON structure without having to define a bunch of coding keys or objects up front etc. Having the ability to pull data through some kind of path macro would be great to see.