There are still some nice buldings in the adventures islands who never found their way in loot and could become precious to give a particualr theme to the home island.
Take for example this screenshot from Secluded Experiments
there are at least 2 new potential building in my opinion
One could be a Pirate Storehouse (with same carachteristic as the spacious storehouse)
And the Ship could become a population building (like a Floating Pirate Residence, I would reccomend at least 100 population thought because can't be levelled up like the Floating Residence)
Adding new buildings to the existing adventure where there are none would make it more exciting to do some adventure that right now are not efficient to do and nobody does anymore (such Sons of the Veld, Wild Mary, Victor the Vicious, Gunpowder,...all adventure snobbed by everybody and sold in Trade Office for 1 coin some time)
Also to put them in sale for gems would grant BB some nice money from those high level who already got it all and can see on a new building like those a way to redecorate with a specific theme (a Pirate theme in this case) their island.