I think something with them (somehow) living in the same approximate area would be good, especially if (former) Slytherin mains would be lumped into that group. It would open up the opportunity for the Slytherins to interact, as it can be quite difficult to pull them in and make sure they're not ignored. Maybe something with the Ministry of Magic offering reduced price housing to certain ages of Hogwarts graduates or something like that. In the wake of the Death Eater trials, certain (former) Slytherins could have their money/family's estates tied up in legalities and not have access to all of the money and, thus, would need to take advantage of the offer.
I think we could get more people interested if a few more details were sketched out for them to see.
And once you're online is there any IM name (etc) where I can contact you? Chatting elsewhere might be easier. X)