I'm making a GUI to make creative mode more convinient. Any ideas on what to put in it?

Some ideas I came up with:
- A day/night slider and weather slider that also has toggle buttons to disable/enable daylight and weather cycle.

- If this isn't a client side mod, possibly adding a flying speed slider/buttons that multiply player movement speed only when when flying to be either faster or slower.

- Maybe instead of making buttons to give command exclusive items, make a separate GUI that you can switch back and forth from that shows your inventory along with some output slots above it that permanently replenish with each item you wish to include so you can just grab n' go (I can elaborate on this if you don't understand fully).

Keep in mind, creative already includes a secret tab you can enable to get a few operator items by going to 'Controls' and enabling the 'Operator Items Tab' option.

Apologies for the delay in replying, anyway here is everything you requested:

- So I'm not fully certain on how to make a slider, it was more of just a concept. If anything, two buttons that increase/decrease the values you want would suffice as well, it's just a slightly less polished solution to what you would be aiming to achieve.

- A client side mod is a mod that you can use anywhere (even on vanilla servers) as it doesn't change anything about how the game actually runs, but rather can just help the player by giving useful UI like a minimap, or shortcuts (like typing out commands for them at a push of a button).

- Lastly, for the 3rd Idea, what I mean by it is that you can have a button in your GUI that switches you to a different GUI with access to your inventory, above your inventory you can then make an output slot for each item you want to easily grab, like this:

You will then want to press on the GUI procedure triggers option at the bottom and create a new procedure for 'While this GUI is open tick'. Locate the block shown in the image below and add a new one for each slot you added, along with placing whatever block you wish that slot to hold.

Once you are done, the GUI should infinitely replenish any slots you made with the block/item you assigned it to hold. When you do close the GUI, it will give you everything that was inside it, so I suggest making another procedure for 'When this GUI is closed' that clears all slots in the GUI to prevent that from happening. Here is a screenshot of what it should look like in-game.

I hope this helped you out, and good luck with your mod development!   :)

You can make hotkeys that take you to any of the game modes, measurements, commands for construction. sorry for broken english(maybe)
 

Amazing explanation Alec Cature, well done! :)