Soulbonding can be written a lot of different ways, coalie. I once wrote soulbonding wingfic, for instance. I think the main thing people expect when they ask for it is two characters who are connected irrevocably to each other, emotionally but sometimes also physically. It doesn't have to be romantic, even, but that's generally implied. There are a lot of ways to approach it, and the fandom/characters will probably dictate how you tackle it.

The soulbonding world is your oyster, basically.