Share the mouse across devices
We already have a way to send mouse input from one device to another. It shouldn't be too hard to implement a more natural "mouse sharing" where moving the cursor past the edge of one device's screen it "enters" the next device's screen from the adjacent side.
This requires capturing the mouse in one device and start sending the input to the other device. It also requires coming up with a way to specify which device is next to which other device, and on which side (probably via some UI).
This mostly makes sense between two PCs.
There are other open and closed source tools that do something similar. Eg: https://github.com/debauchee/barrier