An aesthetic suggestion, a.k.a.: A "This Is Not Important" suggestion.
Make the Synergy Upgrades change the way buildings look like (in the middle part of the screen) after you buy them.
Buying Future Almanacs would style the buildings of both Farms and Time Machines into the the same artstyle.
Then buying Rain Prayer would style all of the Temples' buildings while also converting 50% of the already styled Farm buildings into the same artstyle as the Temples' buildings.