Create a custom UI entirely through tellraw.
It's entirely clientside, allowing you to use it on servers with entirely different software without losing any function!
HUDs, for example, can be sent to players with simple:
/tellraw @s "/correspond hud/<panel w=100 h=100></panel><text x=20 y=38>Correspond</text><text x=1 y=52>clientside UI system</text>"
Or, you can send the player to a custom GUI with only:
/tellraw @s '/correspond/<button action="trigger join set 2">Join World #2</button>'
Check the wiki for full functionality.
