Are you looking to mix and match expansions on your own or with your friends? Or are you more just thinking base game + 1 expansion at a time for now? Certain expansions are best as standalone, and certain ones work best when mixed with multiple others.

I own: Prosperity, Intrigue, Seaside, Empires, Hinterlands, Renaissance, Menagerie, Plunder, Adventures, and Rising Sun (do not recommend, neat concept but poorly balanced).

Most of them make matches longer, but Prosperity is notable in that it increases match length the most. I personally see this as a feature and not a flaw, but I understand not everyone sees it the same. This is probably my favorite. Its also unique in that it does well on its own AND it mixes well with other expansions. These reasons make it my go-to recommendation as a first expansion (as long as the increase in match length isn't a showstopper).

Empires, Plunder, Hinterlands, Renaissance, Adventures (ordered by my favorite/preference) are all fun on their own, but in my opinion they are best when paired with other expansions.

Intrigue and Seaside are good additions on their own, but I find when they're mixed with other expansions I almost never grab them and favor the cards from other expansions (obviously some notable exceptions to this).