You can contribute a VSCode Java Extension to enhance the existing VSCode Java features. Follow these steps to contribute an extension.

You can follow the tutorial here to start your own VSCode extension

Put a server bundle together with your current VS Code extension (in a ./server subfolder, for example), and add the following code snippet to the contributes section of your extension's package.json. Thus, the language server will discover your plugin for later usage.

"contributes": {
    "javaExtensions": ["./server/my.java.plugin.jar"],
}

If you want to communicate with the plugin, add the following code in your extension:

// This should be your command ("my.java.command") corresponding to your server plug-in extension point registration:
function sendMyCommandToJavaLanguageServer(arg) {
    return executeJavaLanguageServerCommand("my.java.command", arg);
}

// This is the VSCode Java command for language server protocol workspace/executeCommand: 
function executeJavaLanguageServerCommand(...rest) {
    return vscode.commands.executeCommand("java.execute.workspaceCommand", ...rest);
}