We haven't decided on all of this yet, we haven't even started the design process.  Mostly we're working on the data at the moment.  Because the data pipeline is the least maintainable thing about metrics at WMF.

Our early thoughts were that the new UI would be so different from the old one that it deserves a new site.  So we were guessing that leaving stats.wikimedia.org intact and putting banners on it about the new analytics.wikimedia.org domain would be the way to go.  Then we can transition the experience slowly and give people a chance to compare with the old site and tell us where the gaps are, if any.  Ideally while we're doing this, the old site would still be updating, but there are a number of problems that could get in the way of that.

As always, let us know your thoughts.  Basically, my approach is that the new wikistats should make it easier to find the data available.  There is a lot of exciting work referenced from wikistats, but I think we can separate that a little bit cleaner from the main work.