Man, I've been playing Witcher 3 since it's launch(a good 560 hours in now)and it has since become the pinnacle of what a large semi open-world story/character driven rpg should be and should strive for. FFXV will be going up against it for me and it has it's work cut out for it!
If XV can reach the quality of deep immersion, worthwhile content, exceptional character development and a well told story it might just best it.
So far, from what I have seen of XV it beats what I've played of the Witcher 3 in combat, possibly art style(need to see more of the world), character design(as much as Geralt is a sexy man beast and Yennefer an irresistible women, I prefer my FF style character designs) and maybe the music or the OST will probably be neck-and-neck.
Tabata, you've play The Witcher 3, you know you have a serious challenger here and you've said XV will BE a challenger. I feel you'll emerge victorious!