Tonguez
A suffusion of yellow
Yesterday I was at a French cafe and on their menu was Japanese Bolognaise - made with noodles and pork(?!) I didnt have that and opted for the coconut curry loaded fries instead.So. The most effective way to "mash up the Spelljammer and Planescape aspects" is to ignore them entirely and build something else from the basic building blocks of "D&D", "magic ships", and "planar travel".
anyway the concept of Mashups isnt to ignore the originals but rather to take the best flavour elements and build something new