With the new improvements to the profile page I think it would be nice to add an additional tab that has a summary of all your profiles reputations/Badges on one screen.
Then you would be able to see that sites specific stats and then see your overall combined rep for all the sites. Just to kinda tie all the sites together a little.
I feel that, for long time users of other sites, this would give them a sense of not having to "Start Over" when they create a new account on a more specialized site, when their question belongs there and not on SO.
This wouldn't eliminate their participation in the site but rather boost it because it would motivate them to maybe make some easy rep/badges to boost their overall Rep/badges.
New users could also use this to get acquainted with other sites and find one that they are most productive in, and have the overall Tab reflect that they tried other sites.
This could even allow for badges to be awarded for cross site actions. When you join several sites, join several sites and gain and keep a certain rep level. This would be new badges they could earn but still have a summary/total for all sites.
I feel that this would create a sense of community among all the SE sites as a whole, such that people would feel as though they belong to the SE family and they prefer SO vs SU or DBA, instead of each site being its own little community. I feel that this is the goal because you already have a communities section on the profile page, that shows all the other SE sites a user is a part of.
Just an idea. Let me know what you think.