Most of these things are possible right now!
Displaying an image can be done with a bot in many ways, the way I do it is using a skinnedcube model with a custom model scale so it looks like a flat screen (it's tedious work to calculate how much you need to stretch it for your image, but it is possible). You can also make click interactions with bots, so a shop shouldn't be difficult.
Now, if you're talking about displaying a png or a custom GUI outside of the game world, then that isn't possible yet, but there's clever ways to mimic similar things - using CPE messages on the corners of the screen and controlling them using server-defined hotkeys. Both of these things you can do with MCGalaxy plugins.
A GUI tint (from what I understand) is also possible, but might be hard due to MCGalaxy not directly supporting the ability to send each player a different texture pack, you'd have to write that functionality yourself. Make two custom texture packs with a different tint, and depending on the player's team, send them a packet with the texture pack with their corresponding tint.
I hope this is helpful and inspires you to create something amazing, but it would be nice if some of your suggestions had a more streamlined process.
