I have spent my last evening and this morning on it. I see three things to do:
1) Make the AI research the tech PlanetaryImprovement sooner.
2) Make the AI build more farm
3) Make the AI build more city.
1) In MasterTechDefs.xml: i have lowered the AICategoryWeight for the techs: XenoExperimentationTech and XenoManufacturingTech.
Added the two tags: <Tags>Growth</Tags> and <Tags>Critical</Tags> to PlanetaryImprovement.
2) In GovernorDefs.xml: added to AIHomeworld, AIManufacturing, AIResearch, AIWealth:
near the top of each list.
3) Added a new Improvement: TOWN, that need the Tech: ResourceExploitationTech and cost 2 food, give +2 pop
City is now an upgrade from town that cost 1 Promethion, and Megalopolis is an upgrade from city that cost 1 Durantium
My result: the AI will eventually build town and city (around turns 75-100).
I really would like others modders to improve on this.