Nibrock's GUI
Currently in the process of redesigning all of the GUI in Minecraft from scratch. Here's a free preview of the ones I've completed so far.
I can't give a timeframe on when I think this will be completed, but hopefully one day.

This pack is 1024x1024 and may cause performance issues?

ROADMAP
GUI (21% Complete)
Accessibility
Bars
Book
Chat Tags
Checkbox
Demo Background
Icons
Options Background
Recipe Book
Recipe Button
Report Button
Resource Packs
Server Selection
Social Interactions
Spectator Widgets
Stream Indicator
Toasts
Widgets
World Selection

Title (0% Complete)
Background
Edition
Minecraft
MojangStudios

Container (17% Complete)
Anvil
Beacon
Blast Furnace
Brewing Stand
Bundle
Cartography Table
Crafting Table
Dispenser
Enchanting Table
Furnace
Gamemode Switcher
Generic 54
Grindstone
Hopper
Horse
Inventory
Loom
Shulker Box
Smithing
Smoker
Stats Icons
Stonecutter
Villager 2

Creative Inventory (0% Complete)
Tab Inventory
Tab Item Search
Tab Items
Tabs

Advancements (25% Complete)
Tabs
Widgets
Window
Adventure
End
Husbandry
Nether
Stone