Gwen and Fergie are similar in a lot of ways.
Both came from a group (No Doubt, Blac Eyed Peas). Quick history - Gwen replaced the original No Doubt then-frontman Alan Meade and FergieFerg replaced another girl who left BEP.
Both are the only female member among adam's apple-attached male members.
Both are still a member of their groups. Both have a particularly themed dancers/posse. (Gwen's Harajuku girls, Fergie's Cholas - not defined) They will accompany her wherever, from attending big awards and appearing in videos.

Both stopped using their respective dancer/posse in their first ballad released song's videos. (Cool, Big Girls Dont Cry) Cant imagine them in those anyway... what would they do? Doing sad, lyrical dances?

Both are fearless when it comes to fashion. (Wacky, colourful outfits and all) Gwen's more of a fashion icon compared to fergie though.
Both injects themes into some of their videos(Sound of Music - Wind it Up, Alice in Wonderland - What You're waiting for For, Chocolate Factory - Fergalicious) Cmon, ran out of fresh ideas?
I think a duet on a happy, fun song by these two would rock the mainstream music charts. Who knows. It'll be another Shakira / Beyonce faceoff kinda thing, but with more fun and less sensual.
*images - mtv.com, nodoubt.com, blackeyedpeas.com, wikipedia.org