Agreed. I like them on SHIELD but they're not major characters so there's no reason why they can't be on both shows, especially if this ends up a mini series.
Maybe something set slighty in the past. We could get some characters that have died back in somehow, Victoria Hand, Zena princess warrior, Tripp... not sure how that would work but yeh. Maybe even a few appearances from Hawkeye. It could focus more on spy gooseberry fool and taking down big villains, something that SHIELD has failed to do, while that show focus on the Secret Warriors theme.