Post Moonlord is unlikely to happen AFAIK, but you could make that idea pre-Moonlord.
Terraria Community Forums
Tachytaenius
Pericles_Corde
I wonder why that is to be honest. I get that the Moon Lord is the penultimate boss. I mean an intergalactic enemy is a fitting end boss. You can't really go further beyond except you cross universes. I will have to see how I write it and make it work. Thanks for letting me know though! 