Now most devices have built-in Bluetooth, so can it be made into an app that directly connects to the PC? Will there be any problems with this?

[zhubinsheng]

[inputstick]

This can be done using Android Bluetooth HID profile.
First problem is compatibility, as far as I know Android Bluetooth HID Profile is supported only by some Android devices.
Second one is that Bluetooth HID covers only some InputStick use cases. It won't work if you don't have admin privileges to pair Bluetooth device, or if you want to remotely control non-PC hardware, like DVRs, POS devices, medical devices, measurement equipment etc.