I think many experts in stack exchange have knowledge in multiple discipline, and they would like to share their knowledge across multiple domains. It is not difficult for them to have knowledge in more than 7 domains. Please make the combined flair shows more than 7 icons to encourage expert supports other satellite sites.
This isn't really needed. The whole point of the flair is to highlight your activity on the Stack Exchange network. Seven sites does that perfectly well. Adding more logos to the flair image doesn't achieve anything because those logos don't actually link to their separate sites. They all link to a single page, which has an accounts tab, which will list out all sites you're using and your reputation under each one. It's simple enough for a user to access that information and access each specific profile without the need for overwhelming them with icons on a flair image (which they may not even know the meaning of).
If you're so worried about showing off every site, try generating your own image. Or, you could even dedicate an entire page on your personal site (like I did) to explaining each site and what you get from participating in it.