1. Be able to zoom in and out in GUI, like the old achievement gui.
2. Be allowed to move the inside part of the GUI around like you could be the old achievement GUI. I feel so limited when I can only move it left and right.
3. Remove the effect when you hover over advancements it darkens the gui. If your going over the advancements fast it can be really annoying/painful to look at.
4. Change the nether icon to netherrack. The two other dimensions are blocks that represent the main block of the dimension, why change it? It would probably look better as well.
5. When moving the advancements menu left and right it should disable the ability to bring up the name of the advancement and its description, it's really annoying when your just panning through them and it brings up the tooltip and darkens the screen.

3. has been implemented since 694ee1d

Gonna close this issue as it is kinda large, feel free to make now issues for any missing or wanted features.

1. #1
2. kinda related to #1
3. Interesting, will look at that
4. Dunno if I wanna chance vanilla things
5. would't that basically be the same as 3, but I see your point