I read this as a fun thought experiment and wrote up a few:
1. Make hub nagivation as frictionless as possible, especially when it comes to mounted interactions and griefing behavior.
2. Never release an xpak without at least one selling point being some sort of new content(NOT a simple numerical change) that will persist beyond the life of the xpak.
3. Less is more, storywise. The more specific and lengthy the writing is, the smaller the world becomes. Build lore not narratives for MMOs, and let the players role-play who they are in this world.
4. Keep arbitrary non-gold currencies used for character progression to under 4.
5. Time gating should be used sparingly and only as needed.
6. Players shouldn't be expected to run external addons and programs in order to compete in the game.
7. The game should always strive to become more educational and friendly for new and returning players. The game should try to offer more tutorial/training/suggestion content which players can reference or be guided towards as needed.
8. Where players are made to compete for resources, there should always be a larger gameplay element relying on that competition, beyond "competing for resources is fun."
9. Large content gaps should be avoided.
10. Probably my most specific opinion that may still work as a generality. Tentpole activity metas should always have some overlap and always some unique space. It doesn't feel good to have to pvp to pve. However, it does also not feel good to perfect one area of the game and have to start fresh in others. Rising tide should rise all boats, but to be "good" at pvp you should have to pvp and vice versa. This should also apply to similar tentpole activities, such as mythic+ and raiding.
...and edit to add one more:
11. Gear should feel unique, relevant, and rewarding. Generic or procedural gear generation may be easier, but it removes novelty and fomo from the game's reward system, and reduces incentivization.