thread could be alternatively named "what the TH games could be like, if ZUN enjoyed higher difficulties and had the means to test these properly"; note that this doesn't necessarily turn TH games into Cave ones; the style (patterns, stages, bosses, cutscenes) would still be different (and having different styles/games means we have more gaming options to have fun with)
Though crossovers (not limited to Cave) would indeed be very welcome

(kind of, you're a drawer, and wants to see how other drawers would draw your characters :p)