Things to implement

OptiFine parity

• GUI support
  • Anvil
  • Beacon
  • Brewing stand
  • Chests
  • Crafting
  • Dispenser, dropper
  • Enchanting table
  • Furnace
  • Hopper
  • Horses, llamas
  • Villagers
  • Shulker boxes
  • Creative
  • Inventory (survival?)

OptiGUI v1 parity

• GUI support
  • Barrel
  • Blast furnace, smoker
  • Wandering trader
  • Trader llamas
  • Loom
  • Cartography table
  • Grindstone
  • Smithing table
  • Stonecutter

New features

• Transfer and fix remaining issues
• New GUI support
  • Camel
• Fix bugs
  • Christmas date bug
  • Pattern-to-regex conversion bug
  • Heights (number range) bug
  • Villager profession range bug
• Others
  • Official Quilt support (requires QFAPI and QKL)

Optimizations

• Optimize code (in beta phase)
• Fix lag spike when a container is opened for the first time (as of now, I blame it on the JIT compiler)

Alpha releases

• In-dev releases
• Only recommended for testers
• Test CI
• API is likely to change
• Start adding support for older Minecraft versions
• ~5 alpha releases

Beta releases

• Generally stable releases
• A few beta releases
• Recommended for average users as "early access"

Release

• Probably same as last beta, when no bugs arise

Full 2.0.0 was not released as it was quite buggy, and #32 introduced a breaking change in the meanwhile.