I bought this game some ten-fifteen years after it was released, after playing the demo and reading all the super-superlative praise about it from all sources.

I played through it recently. It was fun. And, on a decent difficulty level, I played as the Spartans and crushed all the factions I didn't like and forced into "Sisterhood" those that I did. And then, I did the "ascend to transcendence" ending.

And...now what? Is there any fun to play this game again? I already got all the techs and units. Playing as a different faction would just provide something like, different city appearance, and
something like "+1 to research" instead of "+1 to attack".

Is there any fun, different mode of play other than "Different sized map" or "Harder difficulty" ?

There's "Random /Customized map" rather than different sized "Map of Planet"

There's a few scenarios, but I've never played them so don't know what they're about.

Other than that, each time you play the game it starts with you landing a colony pod and you try to conquer the planet, just like your first game.

Despite the seeming similarities you could try a different faction. The faction differences are much more noticeable than the Civilization games IMO. I know there's a few factions I find fit in better with my play style and some I have to alter my style to compete with.

If you don't feel the urge to play again now that you've finished it once, that's cool.

When I finish a game of Alpha Centauri I often won't feel like playing it again for months, but it always ends up being installed on any new computer I buy before too long.

I see. Thanks. I think I will try a harder difficulty after another month or two.

Does chess have replay value, i've defeated my opponent and used all the pieces?

Seriously though, the reason I love strategy games with random maps is because they are (IMHO) the very best games for replay value. If the rules and AI are anygood then each and every game should be different, even playing the same faction / race.

I got back into Alpha Centauri after giving up with Civ5 (it just didn't hold my attention), and now i'm awaiting Solar Empire: Rebellion (another great strategy game with enormous replay value)!

And then, when you want a real challenge, there's multiplayer.