I think it is really cool once you get to the top of the Economy tech tree because the top economy building (the Stock Market) does more things than just increase your economy. Then I thought, "Wouldn't it be cool if more buildings were like that?" So I fleshed it out.

This is what I thought of for secondary mods (note the secondary mods would only apply to top level buildings in their tree):
-Farms
It's tempting to add morale, but that just makes farms too easy. Instead, it would be cool if Advanced farms gave a small economy bonus, since that's what they do anyway (in a different fashion). Flavor reason would be that high quality foods encourage more people to eat luxuriously, which boosts the economy.

-Morale Buildings
Add a tiny bit of influence because it makes so much sense. Flavor reason is that when you have good entertainment, you can export it to other cultures.

-Influence Buildings
Add a bit of economy because influence planets are basically dead planets if you fail to win on influence and you need to revert to Plan B. Flavor reason is that influential cultures are more likely to be business thoroughfares- the places where high powered executives go to make big deals and spend big money.

-Factories
No secondary mods because these are too important and too basic.

-Research Facilities
No secondary mods. Too fundamental again.

-Economy Buildings
Okay, the Stock Market is just too damn good. It needs to lose a mod; I don't care which (between influence and morale).