I'd like to complete my museum and that's it. That and paying off the final debt are kind of what I consider "beating" the game in older titles. Maybe I'll try getting the achievements but I don't really care about DIYs I don't need, or maxing out the catalogue.
I consider complete to be full museum, happy with island decor and happy with house decor. I don’t really care about nook mile achievements or catalogue because nobody can see them
While I would like to complete the museum (have the art section left) and nook miles achievements, the lame but true answer is that I will feel my game is done when i open it up and feel like I have nothing to do. I love the open ended ness of the game that doesn’t force me to do anything in particular, but assume eventually I will run out of that excitement to open my game each day so I can chat with my villagers (I love decorating but the villagers are what really make me love the game so much, and they are the best part of the experience for me) and whatever little chores and decorating I am in the mood for that day.
and when I feel tired of the game I won’t feel like I need to push myself to play just to reach my own somewhat arbitrary goals.
of course I have not gotten bored after 1700 hours of playing, so it may be several years (if ever) before I find myself at the ‘end’ especially if they keep updating.
having said all that…I would also really like to eventually breed my own blue roses, through the lazy, not very rigorous, attempt I am making…but we’ll see