The idea that the PCs be a group of (aprentice?) Wardens of the White Council makes sense - certainly an excuse for a wizards to hang out together.

Another excuse might be to have a group apprenticed to the same wizard (like Harry & Elaine)

However, PCs could also be part of Harry's "paranet" as alluded to in the sample chapters of 'Small Favour' - new, young, feeble magic users who have banded together for mutual protection.

Or, what about a group of half turned, Fellowship of St Giles types?

Or, changelings a la Lily, Fix et al in Summer Knight?

Or what about the Alphas as a template?

Personally, I like the paranet angle as overpowered PCs are kind of unimaginative to play - it could be fun to see them develop, come to the attention of the White Council etc.