I'm quite new to minecraft modding and I was wondering if it was possible to override a vanilla menu, more specifically, the server selection one. If it is possible, how wouldI do it?

 

I want to do this to add new functionality to that screen. Any response is appreciated!

It's possible, but difficult for a few reasons. What are you wanting to add? Can it be added in-game or does it NEED to be in the main menu?

 

Generally it's frowned upon to alter the existing menus as it:

1) Requires a coremod. (Which are last resort.)

2) Only keeps the last mod to change it's changes.

3) Can just be plain annoying depending on what you're changing.

 

But if you're only adding functionality and not removing any and you're not too intrusive about it, people might be willing to help.

I was looking to add an indicator next to the server's ping graph that would show the icon if certain conditions were met.

You can use the GuiOpenEvent, in which you get a GuiScreen object. You can use that object to check if it's a instance of the vanilla menu (GuiMultiplayer in your case) and then set the GuiScreen object to your new class extending the vanilla menu, so it still looks like the main menu instead of remaking all of it.