That seems the be the main problem. They had rewards that were unique and desirable, and then they all got added to Events xD
In regards of unique building buffs. There has been no shortage of proposals. Like the long forgotten and abandoned
GB contest, or settlements themselves. I still strongly agreed with you. That just buffing them won't futureprove them indefinitely. An unique or rare* buff would be.
For example an unique but somewhat useful buff could be reduced time for exploring new provinces/continents/AF&OF ports and expeditions on the space age cmaps. This can help speeding up advancing on the cmap and speeding up ages from AF and onwards. Not game breaking but at least it'll save time in the long run. This is just an example to show options outside of regular resources and as prove of concept that the game still can see more unique stuff.
If they really wanted, they can boot up an global contest. Letting players coming up with unique buffs reserved for settlement building's "special" level(s). Than arranging an top3 unique buffs for each settlement and letting players globally casting votes.
*rare for example handing it out sparsely. Like %
on forgotten temple. Applying a little more %
on one of the 1-off settlement rewards. The supply is still heavily constrained, making it rare
.
