The Imperial Palace seems like an excellent placeholder for this.
Conquering an Imperial Palace by force should turn it into a War Museum, providing a stacking 5% bonus to troops and ships.
Conquering one by diplomacy should turn it into a Peace Museum, providing a stacking 5% bonus to research and trade.
Capturing one while you lack an Imperial Palace gives you an Imperial Palace (and maybe that should get a better bonus, or explain the consequences of lacking one better)
Of course, War and Peace Museums count as Palaces when captured.
For topics that do not fit in another sub-forum.
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