How to decide on how big of a canon-divergence you want to do?

I'm writing an oc-insert fic right now, so there's obviously going to be some canon-divergence. That's unavoidable. So far, there has been a few here and there, but those are very small and inconsequential.

If I'm going to keep writing, then I'll have to decide if I want to go big or not, else I can't continue. But I'm just so unsure...

The 'go-big-or-go-home' mentality is there in my head, but if I actually go 'big', then it's going to be a lot of work, and I'm afraid I won't be able to commit to that since I'm the type that write as I go along with vague ideas floating around.

How should I go about this? Is this a common problem everybody else has?

Edit: Thank you for your replies :D