What is so special about transistors? There are many non-linear active devices that are relatively unexplored due to the local maximum of transistors. How about homemade flame triodes or magnetic amplifiers or memristor.. I think you are going to love these guys:
Just an example; film deposition is useful for all kinds of things, including producing solar cells, but a single BJT or FET seemed like a fairly simple starting point.
Thanks for the links!