Ability to raze cities

It's already in the game, just ransack the city center

Maybe half of the population could be moved to your cities? B

Empies would often move populations from homelands to other places.

What I've been doing is right before razing, I'd do something to use the population... I'd either sacrifice the population to build, or if i already have researched the tech that disallows it, I'll just conscript the whole population and use them to raze

alternatively, you can make the city independent, have them build, make them a puppet state, and when you're ready to assimilate them, just repeat the above

I would favor city ruins similar to the existing outpost ruins. if someone can defend the terrain the ruined city's districts should still exist