This is an example Bukkit plugin for demonstration purposes on how to add custom tooltips (on-hover) over commands completion suggestions. Made for Paper 1.16.5
This presents two different approaches: one using ProtocolLib by listening to the TAB_COMPLETE packet and the other using Paper's experimental Mojang API AsyncPlayerSendSuggestionsEvent.
Both of these cases are baked suggestions, a bit of a more complex (expensive) mapping system can be implemented with very little effort for dynamic suggestions (e.g. online player names, factions/towns listing, permission nodes, etc.).
