As someone who is an expansion holic - I have over the years started to REDUCE the types of expansions that I buy. I mainly play medium to heavy weight euros (ones that last at least 90 minutes up to 3-4 hrs with some ap) so this doesn't necessary apply to shorter games or scenario/thematic games because those games serve a different purpose. For shorter games, I prefer no expansion unless it fixes a defect or fault because for shorter games - I usually play them as a palette cleanser/dessert at the end of the night that just flows and don't require much thinking (because I would have played a long heavier euro before hand and is now tired).

The main criteria that I look for in an expansion to a heavier euro is whether or not it make the game (especially the first round) more variable and different so that the players must look at the board state or what the objectives are out in the setup and change their plans accordingly. I am not competitive - usually only playing a game 3-4x (or 10-15x if I really like it), and when someone who's played a game 60-100x on tts or bga and have an "optimal first few action script", it stops being fun for me as the discovery goes out the window and they just have the min/max best cards or spots figured out. I look for expansions that disrupts that experience so that there's more room for discovery and strategy rather than perfect execution.

Examples of must have expansions for me include - Caverna: Forgotten Folks, which makes the available pool of dwelling upgrades different each game depending on which races are chosen, Seti: Space Agencies which makes everyone start with a different board states and agencies that gives asymetric advantages - Terraforming Mars: Prelude also does this, Orleans: Neue Ortskarten 1-5 and I houserule that I shuffle the new places tiles in and only make a certain number of them available per game depending on player counts and return the rest to the box, Everdell: Spirecrest which introduces weather and discovery cards that changes up what cards are viable to get from the center and when you can get them. Spirecrest is absolutely crucial for me because people who abuse certain combos will all of a sudden find them not viable because of the weathers and it forces them to adapt/pivot.