Forget workarounds using a horse, here's a proper mod that lets you play The Witcher 3 (well, most of it, at least) in first-person.
skacikpl's mod (which is still a work in progress) is fairly easy to get running, and once enabled lets you walk and run your way around the countryside in full first-person mode. You can even see Geralt's feet as you go.
Given the fact The Witcher 3 was coded as a third-person game, it's not perfect; combat doesn't work, for example, so the camera pans back out when you draw your weapon. It doesn't support first-person horse-riding, either.
But for those like me who just wanna run around in the grass and soak up the wind and sunshine and just how pretty this world us, it looks awesome. If you want to download it and give it a try, the link is in this thread.
Oh, and before you put 2+2 together, this doesn't work in VR and the guy doesn't have a VR kit to develop on, so this is a strictly traditional first-person mode for now.