Honestly it feels to me that Civ1 - Civ2 is the most direct upgrade in the series. Civ 2 was mostly just a better civ 1. From civ4 onwards, the series was a lot more willing to shake things up in its gameplay.

Civ 2 was without doubt a much uglier civ 1, though. Isometric graphics in win 3.11 wasn't a good bet.

Civ 1 had good pixel art (look at those mountains! Not to mention the intro), good colors (and more of them!) and clean iconography. For me the look was part of the magic, so I never got into Civ 2.

I considered Alpha Centauri as the sequel, both in the continuation of Civ 1s final goal and the expanded gameplay.

Civ 3 already started to shake things up.