It seems like most 3D MMORPGs would fit your description, tbh. What you consider "fun and addicting" may vary. In terms of customization and drops you could consider checking out Aion or Perfect World - they have very extensive character customization and appearance modification. And you can fly
Another game I found to be decently fun was Eden Eternal, although it's more technically a 2.5D game with 3D graphics.✧
Every (or almost every) piece of armour is paintable in whatever colour you want (as long as you buy the paints and they can be bought with ingame gold, No need to spend IRL cash) and it meets all the other criteria you specified. Give it a go.