Hopefully, maybe the automatic tail or a way to heal it. Honestly I think Hiccup with leave with the dragons considering the bond he and toothless share.
Also another thing to point out with most hybrid animals between two species... they tend to be sterile unless they have enough chromosome structures between both animals. Its why wolves and dogs can produce offspring that aren't sterile and why donkeys and horses produce mules that art. Biology is so weird and fun at the same time.
So light-fury night-fury hybrids might have a chance to be sterile unless they are like wolves and dogs or house cats and servals.