Here are some cool free unreal engine plugins that I would recommend you try.
1: content made by epic games – they have lots of free things that might be cool in your games.
2: WindowsDialogBox – this plugin is simple and lets you make a windows pop up.
3: Auto Size Comments – this changes the size of comments in blueprints when you add or move code around.
4: Android Play Billing – this may be useful if you are making a game for Android.
5: Firebase Analytics – you can use this to get a lot of data about your users like how much they use your app.
5: Text translation – simple plugin that translates text.