I came up with 2 random HUD ideas a while back and thought I would share them here! From what i know of forge and java and Minecraft itself I dont think one of these two ideas would be possible but hey, thats just from the extent of my knowledge.
A way to make the toolbar (along with the health and stuff) sway with camera movement. Basically, you move the camera to the right and it takes a small amount of time for the toolbar to move back into place. I was thinking having it have the same sway amount or whatever as the items you hold so that it looks a bit nicer. It would also be nice if other HUD mods would move also with this.
A helmet that covers part of the screen only when in first person. This idea came to me when i was playing DooM with the Ultimate Doom Visor mod and though "why isnt there something like this for minecraft?" Basically, if you have a helmet on, then it shows the helmet on the screen, and depending on the material the helmet is made of will change the helmet also. This one may be a bit harder to pull off though. And if it is possible it would be nice to have it work with another mod that allows the camera to have an "over the shoulder" view and not show when that is on.