Twestival is an event that has impact. Every year, on one day, people all over the world come together to make a change.
The tag-line on the Twestival website - http://www.twestival.com/ - is "Tweet. Meet. Give." How about turning that into something like "Meet. Ideate. Change."
Twestival is very focused on raising awareness and funds for charities. But the idea could be altered into global workshops to realize concepts.