It's probably Portal 3, or something completely out of radar but can be considered a small scale project but unique in it's gameplay (?),
I can't think of an IP they would make that would fit perfectly and run like a dream on their hardware than a new Portal 3 - the least demanding IP I can think of that could sell hotcakes.
But again, one must not forget, Valve would likely never do a Steam Deck exclusive game. Makes no sense to bar a game on their steam store that is unplayable on the PC.
Regardless of any game, whether a new IP or not, if it were to come out - the question of whether it would push hardware sales is an unknown outcome.