I know it's a hat, but the mask design could replace the disguise kit's masks. Whether it was just a re-skin or actually had stats, I thought I'd throw this idea out there!

Personally, I'd love to see a disguise kit that lets Spy fire his disguised class's primary weapon. These holographic attacks wouldn't be visible to your team, or do any actual damage, but they'd really sell a disguise!
Of course, savvy foes could still notice the discrepancy.