Character being done means it hitting the cap (optimally min/maxed) on the respective build.
Player being done on the other side may mean the things you listed and that’s a huge topic.
If we could theoretically design an aRPG that’s making it so that the player never stops having fun while playing with a character, we would have realized the alchemist’s dream turning pixels into never ending gold. All of what you listed would help for that:
- Long progression
- Fun gameplay
- Good itemization
- Goals to reach
I personally think “the key” is hidden behind the last of the above. If we can always “supply” the player with mini-goals, he would always play. But these goals have to be different and dynamic (not some static repeatable daily quests) and have to depend on the player’s decisions during the adventure.
Said otherwise - we need an AI. We need to learn/know/remember what the particular player considers fun in the game and we need to keep the fun for him going through the AI. This would of course mean that after say 1k hours your Diablo experience will probably vastly differ from mine and it will be practically impossible for these to be compared aka there won’t exist a competitive aspect outside of a Challenge Rifts scenario, but we’ll both have much more fun in the process of slaying monsters, exploring the world and evolving our characters since the AI would supply us with what we want.