To the alliance overview window you can add a new tab, where players from that alliance could build the new building (lets say, they could only build if they are located in their town, like church), founding towns of alliance could then every 24 hours activate a buff depending on the level of the building (or if you want it to become more grindy, make it so that players need to contribute products (as for travelling fair, forts), to start building & then to activate a buff).
make it have infinite levels, same as church, but lower the required labor points and XP gain, to possibly prevent low levels not be able to build it (then there comes an opportunity to create achievements for it, new ranking page etc)
if its not feasible to activate a buff for only certain players, then it could be made as a consumable item, that everyone could redeem if conditions are met (24hrs passed before last redeem etc)
there are many possibilites to implement this and the only limitation is InnoGames themselves. I bet if you let the scripting community on this, they'd give you a working prototype in no time
... but alas, that wont happen and we can only speculate about new additions such as this, which is a shame. So much potential in this game and its being wasted every single day