Goldberg: The Man
Seth: The Real Man
Reigns: The Realist of Men!!
That's a god question!
In all seriousness - Im not sure. Reigns is a contender. Taker is another one. Super Cena maybe?
In my eyes Goldberg is like Brock. If you put someone like AJ in front of Goldberg it will be like putting someone like D Bry in front of Brock. AJ & D Bry are brilliant wrestlers, however, Brock & Goldberg are just beasts, and them competing with one of the 'smaller guys' could be considerably tricky and hard to sell.
IMO at Mania if Goldberg is to have a Mania match I can see it going 1 of 2 ways:
1) Roman VS Goldberg - if WWE wants Goldberg to put Roman over
2) Cena VS Goldber - if WWE wants to do something like a 'dream match you never thought you'd see'
A curve ball which WWE could throw is Bray VS Goldberg I guess, if they wanna give Bray a push. I can see it now - the Wyatt family ambushing Goldberg and Wyatt cutting a promo along the lines of "You think you are unbeatable Bill? You went 173 matches without being defeated but that's because you never had to compete with the eater of worlds!"