I've made a simple packet API plugin as an alternative to ProtocolLib. It uses the Bukkit event system, so you can just register your packet listener the same way as any other listener. The events are called synchronously, I might add async events in the future if people actually use this. Example code is in the readme file.
https://github.com/GitFyu/PacketAPI
https://github.com/GitFyu/PacketAPI
