Too many people would get confused, forgetting to substract 1 from the BC-tagged number, or unsure whether to add or substract 1, or wondering if the other person remembered to subtract 1.

I suggest adding 7000 to all dates, as well as using negatives. So...

2010AD -> 9010

1AD -> 7001

1BC -> 7000

31BC -> 6970

5000BC -> 2001 (except people talk about year ranges, not years, when refering to events that far back, so there'd be no confusion with our current AD references)