I'd suggest something completely different:
A collection of museum rooms that can be put together in one bigger set, and all of them using old playmobil items from discontinued themes and sets. That way it would be an actual playmobil museum, featuring the history of playmobil.
The medieval room could feature a stand displaying a nuremberg helmet, chest piece, old sword and nuremberg shield, maybe along with a flag and drum.
Same room could also feature equipment from the 90's, such as tower, lion, fleur de lis and swan shields and equipment.
Maybe a tapestry to the wall featuring an old diorama.
Other wings could feature western, victorian and pirate things from older sets.