If I were to create new GBs and attributes, I'd go for these:
1) The Bermuda Triangle
After collecting this, the first fight right afterward could be a random one*, only in manual mode, and the player would be led to a preparation window in which he'd pick his units, always against a 2-wave adversary. So, the player would only be able to fight with corresponding age troops. For example, if the adversary is from the Modern Age and I'm in Titan, I would need to have Modern age units to fight.
After a victorious battle, I would have a chance* to transfer any item from my inventory to any other world from the same server.
So, as the GB levels up, the chances of the random fight happening and the chances of being able to transfer something would increase, up to 49% no matter what.
This way people would be able to transfer finish all supply productions to a primary world, transfer a wishing well to a secondary world etc.
* chance of just not happening.
Maybe 5x6
To reach a new level, these would be recquired:
- Promethium, supplies and forge points
2) The Temple of Salomon
This one would depend on other Great Buildings, as it would have a chance of providing diamonds, acording to the number of Great Buildings in your city. As you collect, it would either provide coins (as Salomon was filthy rich) or 10 diamonds per GB in the city (including it). As the GB's level goes up, the chances would lower for coins and would increase for diamonds, with a limit of 70%/30% no matter what.
Maybe 6x4
To reach a new level, these would be recquired:
- Orichalcum, medals and forge points
3) Black Hole Rotor
This one would enhance all other Great Building's collectibles. It would start with only 1%, and on level 80 would reach about 10%. So, The Blue Galaxy capacity of generating medals would increase, so would the forge points of The Arctic Orangery, so would the treasure goods generated by the Ark etc. If the GB doesn't provide any collectible (like The Statue of Zeus), the percentage would revert to its attribute.
Maybe 4x4
To reach a new level, these would be recquired:
- Treasure current age goods (yes, founders would have to approve of a new level for each player), current age units and forge points