Double Agent.

/Very old idea from my drafts./
As you can see from the title, Spy will be double agent...for both teams.
It means that this unlock (Then I thought that there would be unlock for the Disguise Kit; now it's only the idea) gives Spy ability to switch teams in combat.
Here are versions how it could work:
1a)fake killing teammate
1aa)DR for the mate (he becomes invisible for a few seconds)
1ab)mate-corpse is on the ground (until time is over OR he presses M1)
1b)real killing teammate (+1\-1\0 points to your score)
2a)there is no FF(friendly fire) for the Spy
2b)there is FF for the Spy (full/ partial)
3a)changing team-color (ТAB doesn't show) like you switched teams
3b)changing team (ТAB shows)
4a)fake killing "teammate" (=enemy)
4b)real killing "teammate" (+1/-1/0 points to your score)
5a)there is no FF for the Spy while in enemy's team
5b)there is FF for the Spy while in enemy's team (full/partial/increased)
 The best version I think is: 1aa(only if you use DR)-2b(full/partial)-3a-4b-5b(increased).

Spy-checking doesn't work properly on these Spies - though they take damage - because Spy can be shown as a Spy of your own team with any mask on his face, even he can use weapons(fake killing his "enemies") plus drop his disguise like he's your teammate-Spy. He can't destroy buildings while he's "in your team" (anyway, he can't destroy buildings of his team, but he can use fake sapper). To switch teams, he must kill teammate (real or fake, depending on color). It's so easy to be killed by your teammates' weapons. Double Agent is always risky job. But this new playstyle for Spy is worth it, I think (that's why I like this idea so much).
(And remember: your imagination is better than details I've written here. You're welcome into comments!)