I fully agree that there's room for such things at Avebury. I would say that not every Christian goes to Bethlehem each year nor does every Muslim head for Mecca each year. Even if you make somewhere you spiritual centre it doesn't mean the whole faith has to go there for every celebration.

As to the amount of stones needed at a new site ... that's for the builders to decide. It's part of he responsibility of organising a faith. You could make it mark out every important date in the calendar. There must be some pagan lawyers out there who could draw up a bit of paper to get around the insurance thing. If people are serious about doing things for their faith and something happens to them while they're building a temple surely that's a message from their gods and they probably got something wrong ;-)