There is the PlayerChatTabCompleteEvent to modify the tab-completion of a chat message the player is typing in. However, there is no way to modify the tab-completions of a command the player is typing in. Plugins can modify the tab-completions of their own commands, but there is no event for it so that all plugins can modify all command tab-completions.

Example:
I am creating a login plugin, and until the player has successfully authenticated, he shall not be able to do anything, so he shouldn't receive command-tab-completions of commands either. After all, such completions might contain information that should only be visible to a specific player - and such data shouldn't be sent to an unauthentificated player.