AI city spam: they are hardcoded to produce a settler first, then decide where it should go.

Increasing city distance can cause greater issues for the player. Decreasing it will delay consumption of their space, but make it worse when they come for yours. Loyalty may have been designed to deal with this, but it is the burden of Firaxis that automated site selection is so poor; don't make it mine—the city should never have existed in the first place!

Core change: New Era Settler.

A less herculean solution than changing behavior is to limit how settlers are generated, resulting in 9 per player across a full game. This is still double what I tend towards, and keeps it from feeling like there are no good spots left later. This change also does not account for cities you conquer, and thus rewards aggressive play. A choice to raze will last longer, and if they do come back there is still attrition.

The rest are designed to further develop this line of thinking.

Settlers Retreat: This is actually meant to help the AI; remember, they're stupid. It's up to you whether casual theft, and the grievance cost of surprise war, are worthy strategic options under this new scarcity. I personally felt coerced into needing to capture, and that it was a waste to try using diplomatic points. Also, without protection the AI may break from other mods which trade their starting bonuses for better stats later.

AI Deity Settler and Smooth Scaling: Such as these.

Strategic Forts: The AI will still come at you; this renews your ability to push back. Just remember never to grant open borders, as the base game has no way to revoke that until you declare war.

Barbarian Clans: I love city states. I hate immediately killing them for the same reasons as an AI, when I'd otherwise enjoy trading with them. What I do instead is add mods to this mode and set the initial city state number between 1 and 3 (don't do 0; it disables conversion completely), letting them fill out organically starting around the Medieval era. This stretches out the introduction of mechanics to worry about, making later play more engaging.

City State Introductions and Defense: With the paradigm shifted to effectively "growing" them, this makes it a little harder to abuse or to see none at all.

Religious Settlements: I don't really have a problem with specific extras, just with having them too early. Classical era still comes quickly enough.

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Still looking: alternate use for settlers.

With a couple conquests, I don't see myself holding more than seven cities; on top of city states, it's too much to manage. What I'd really like, and may make my first mod for, is to add a Return to The People ability which sacrifices a settler for an even number of great person points across each type. The AI will use this ability if a settler is still alive after some number of turns.