I imagine that if he really wanted to, he could create a different genre of games still set within the Touhou universe (right now it already includes two very different ones. Fighting and Bullet-Hell)
If he wanted to create a new IP, it would be for a reason separate from wanting to make a different type of game. Because any type of game genre could easily be adapted to the Touhou universe, as shown by all those various fan games.
That said, the Touhou-verse itself is malleable enough that it would have to be some really off-the-wall inspiration for it to be completely incompatible with Touhou.