This question already has an answer here:
- Can we have cross-site badges? 4 answers
It might be useful to encourage existing users of stackoverflow to cross pollinate other stack exchange sites. While rep-sharing is not useful, as rep is meant to be a measure of expertise in one subject, badges may be one way to encourage participation across many sites.
This was triggered by seeing how few people actually had greater than 1k rep on all three main sites (SO, SF, SU) via Dennis Williamson's Stackathlon app. I had tried to do this by hand prior to the API under the assumption that many people would be answering questions on all three sites, but it turns out there are only 33 people, of all the users of stackoverflow, that have a 1k+ reputation on all three sites.
Over time it may be useful to provide badges that appear on all sites one is participating in that are awarded for extraordinary participation in the whole universe of Stack Exchange sites. For instance, a silver badge might appear on all my profiles if I obtain 1k rep on 5 or more sites, and a gold badge if I have more than 3k rep on 5 or more sites.
I would hesitate extending this much past 5 sites as we don't want to encourage spamming questions and answers on sites where the user has little interest, but across the universe of stackapps I know I'm proficient or expert in at least 10 current sites, and while it's not a big nudge, the idea that I can get extra awards - especially ones that appear on all sites I'm in - would move me to participate a little more on sites that I am registered on, but don't visit regularly.
I expect, eventually as sites are launched, most users of any one site will have some interest or knowledge in at least 5 other sites. This may provide the extra nudge some sites need to maintain an excellent rating in terms of usage.