Since it seems S5 is tying up the loose ends, do you think Spike will get some development in S6? Maybe some info on his origins, or maybe he grows up?
We know his origins. He was an egg. Twilight hatched him with magic. That pretty much sums it up. As for character development, I'd say he's been pretty well developed into the show's Butt Monkey.
Well, in-universe, less than two years have passed since the events of the first episode. Considering how long dragons live (long enough that they take hundred year naps), I think it's reasonable to assume that they don't age very quickly.
I think the problem is that the writers don't really know what to do with spike, where to go with him. So I see little hope for any more character development for spike, though I'd love to see some!