I don't think this is the best idea, though it's certainly better than what those GBs currently do. Maybe we can come up with something completely new for them. And I'd include Notre Dame as well. Maybe even ditch the Happiness boost, too. Coins, Supplies, Happiness - the three resources you're most likely to have more than enough of.
The Colosseum and the Capital are useful for a little while, but as you go up through the ages, and acquire more expensive expansions, they become useless. But ND, SN, and LT are useless from the age when they appear. And IMO, so is the Oracle of Delphi.
So what could a new GB function be?
*********** Active: Random Goods from the previous age, as mentioned in the OP. ***********
* Passive: A boost to other GBs?
* Passive: Reduced Scouting time/cost?
* Active: Guild Power?
* Active: Random Fragments (SoK, WW, Shrink kits, etc.)?
* Passive: More Incidents?
* Passive: Bonus to Incidents?
* Passive: Boost to Guild Expedition chests?
* Active: Friendship Tavern Silver?
* Passive: Boost to Friendship Tavern?
* Passive: Armor to military units? Say, a 10% chance of one unit having armor at level 1, 11% at level 2, up to 2 units at level 3, 12% at level 4, up to three units at level 5, etc. Or some kind of progression like that.
Just a few random thoughts off the top of my head.