The difficulty in makine a sequel, is that you want to change enough to warrant it - and not just pull a FIFA, re-selling nearly the same game each iteration - but you do not want to "lose yourself" - you're buying "Civilization something" here, not another franchise.
Keeping the core game, but innovating on it. That's the hard part.
One very interesting thing in the Civ serie is precisely that they managed to always change things, but "stay Civ".
I started with Civ I, right from the nineties.
Funny thing, Civ3 is in my mind the "less polished" of the serie, the most flawed... but at the same time, it's the most immersive and the one I spent the most time on.
Civ IV, on the contrary, I consider it to be "objectively the best"... but it still can't grasp me by the guts as easily as Civ3 could.