[DrPraetorious]

I've been thinking along similar lines. I don't see any effect numbers that outright change population, income or resources, or that add sites - but that doesn't mean they don't exist.

Effect 10034 - random event - seems like the most promising, as you'd think you could generate the various beneficial events with it.

I'm busy testing other stuff, but if you want to try the following spell:

#addspell
#name "Event Test"
#descr "Event Test"
#effect 10042
#spec 268435456
#school 0
#researchlevel 0
#fatiguecost 0
#path 0 4
#pathlevel 0 1
#damage 11
#end

Try changing the damage value and see what events you get.

You may need to change the value in #spec back to 0. I'm not sure what it does.
Raging Hearts, Wolven Winter, Black Death and Blight have #spec 268435456

Baleful Star, Hurricane, Volcanic Eruption, Rain of Toads, Locust Swarms and Tidal Wave have #spec 0 (or #spec 8388608, which just lets it work underwater.)

Anyway, I'll test this myself eventually but there's some combat spell stuff I'm still trying to figure out.