In an interview with Gamasutra, Yuji Naka has explained why he decided to leave Sega and set up Prope.
He says that he wanted to "get into the nitty-gritty details" of games instead of "looking at every game the company was developing".
Talking about plans for the future, Naka says that Prope is "planning on making a game like Sonic right now". Hopefully this doesn't involve tapping and penguins. He also wants to continue the Let's... series if they sell well enough, however Let's Tap has seen abysmal sales in Japan.
Why can't anyone see that we would rather play Sonic 4?