I noticed that the amphitheater, museum, opera house, Louvre, and palace didn't simply have empty great works slots that you could slot anything into. Even when unslotted, they displayed a specific graphic.
For the amphitheater, it was the inkwell. For the opera house, it was a musical note. For all the rest, it was the palette.
Also, not that Grieg had a strength of 120, while Bruegel did not. No telling what that's supposed to mean.
So, there's really more questions than answers here. We don't how to creat these great creators, and what you can do with them if you don't have the right type of building. Options seem pretty thin right now for great musicians.