There doesn’t need to be one. Any windows handheld can run Microsoft’s first party games.
Microsoft are having enough trouble with home consoles, we don’t need them spreading themselves thin with an underpowered handheld too.
A mate of mine has the ROG Ally and while it’s a damn nice device, when playing uncharted 4 on it to show the performance to me it chewed through like 25% battery in about 15 minutes. You can’t have high performance handheld while having even remotely good battery life.