Based on the previous overall goty winners over the years it seems like whenever a large open-world western RPG of exceptional quality comes out in wins in a landslide, with that in mind my bet is for Witcher 3.

Zelda U and Uncharted 4 have the highest chances to win GOTY 2015 if you ask me!

I think there's 3 games that have a really strong chance... Zelda U, Uncharted 4, and MGS5: The Phantom Pain... All three of them are expected to be phenomenal games. I will probably go with Zelda U out of the three, because of quality control.

What was 2014's Game of the Year anyway?

Right now guess Uncharted, but could see Witcher or Batman taking it too.

Xenoblade, or Zelda, or Bloodbourne

It was Dragon Age: Inquisiton. Smash Wii U and MK8 were close behind I believe.

Think you're being far too hopeful for a JRPG on that first one, looking through the GotY blog thingy there wasn't a single year that a JRPG even got all that close to winning the majority of goty awards as far as I could tell.  

Also, on the question of who won 2014 here's a link:

http://gotypicks.blogspot.co.uk/

Basically it goes Dragon Age way way ahead at 98 awards (only counting site staff awards), then Middle-earth at 44 before you get to Smash and MK at 20 each

Honestly I'd have to go with Zelda U or Xenoblade Chronicles X. But you know, those games come from Nintendo. So they probably won't win.

Bayonetta 2 was also pretty close.