Though the concept/idea is rather interesting, I find it kind of distasteful as far as the current story has been concerned thus far.
I think some people may be taking the whole thing too literal. Also, a lot of the things may be metaphorical (such as when he is having the dream/nightmare and the demon face shows a resemblance to Guts). I am more inclined to side with the fact that Guts has inner demons (which is obvious), rather than being a physical demon. I just think it kinda kills the mood of the development of the character (at least for me). As far as the physical appearance of Guts, I don't really see the resemblance there. He has looked like the other human characters as far as I can tell, but maybe I'll look a bit closer next time.
Plus, I figured he gained a lot of "strength" from being in the intersection (or whatever it is technically called), between the worlds, for so long. I think I recall reading someone in a chapter saying that, proposing the idea of it. (here
http://grab.by/5op6). Though it is just a proposition/thought, I feel that it is more tasteful than him being a demon himself. Of course, it always open to anything really.
Also, Zodd is fixated on Guts because Zodd's whole deal is fighting with the strongest opponent he can find. As of now, Guts is one of them (as well as SK). I don't think it has anything to do with Guts being his offspring.
I won't discourage or ignore the idea of it, but I just find it to be too speculative (hah, I dunno how far we can take this idea, or any other for that matter, since it's still a work-in-progress, and our world isn't "Berserk" world, so it's hard to actually make strong speculations). I would just be more pleased without any demon-human inbreeding associated with Guts; takes the flavor out of it. But then again, anything can happen. Either way it turns out, the overall story will still be amazing.