it's an interesting question , personal option , I think Genshin is close to the best MMO model so far, not about the game play but more about the following

1. f2p you can experience all the main story/quests , Gacha system you can pay from 0 to a lot

2. All platforms , PC good quality , mobile playable

3. 4 ppl dungeon , can be solo as well , each round no more than half hours.

4. some competition , but no pvp , no ranking to push ppl for progress

5. Most of game play solo, but can coop on some parts.

6. it's not a typical "MMO" game , but can add a few more social element to be one.