Are you level 60 yet?
If you're not, then save up for these:

• Riding skills, so you can move around faster and fly.
• Vendor mount. The Traveler's Thundra Mammoth is 20k gold, it has a regular vendor and a repair vendor. The Grand Expedition Yak is 120k gold and has a transmog npc and a repair vendor.

Don't spend gold on gear if you're not level 60, you'll replace whatever you buy along the way.
If you're level 60:

• Find out what legendary gear is appropriate for you, and save for crafting that.
• Are you interested in dungeons above mythic level, raiding or rated pvp? then use it to buy consumables (flasks, potions, food, enchants, gems), find out which ones are best for you and get enough to cover your runs.

Regardless of level, are you interested in professions? Then use the gold to get the recipes and level your profession up.

Other than that, there's a lot of stuff to spend your gold in. Appearances, toys, mounts, pets, old world recipes.