GPL-Style License w/ A Twist?

txsable asks: "I'm trying to find out if there is a GPL-style Open-source license available with a special twist: that any modifications made, while being allowed to be released as per the terms of the 'normal' GPL, must also be submitted back to the original author for possible inclusion in the main project. I know it seems ridiculous to require by license what should be a common-sense procedure for open-source developers, but I've seen enough projects which were greatly modified from the original project, with features the original project either was working on or had also developed, and it led to confusion and/or unnecessary duplication of effort. Anyone have any suggestions for me (other than writing my own license...I don't speak enough lawyer to make something that reads as bad as a legal document)?"