Hmm... What about this:
Each team has 1 randomly decided traitor. That traitor's victory condition is that the opposing team wins. (Stays the same)
Global gold will affect the 4 members of your team and the 1 traitor on the enemy team
Global buffs will affect all 5 members of your team including the traitor to avoid unintentionally outing someone
At the shop, the traitor can buy certain benefits for the enemy team. Here are some examples:
Ward Coordinates: Grants the enemy team vision of your currently placed wards. This vision lasts until those wards are gone. This does not reveal wards placed after the item is purchased.
Gold Stash: Steals up to 100 Gold from each member of your team and gives it to the enemy team. The gold will be taken from all other teammates and only affect loyal members of the enemy team. If a player has less than 100 gold, it will take all the gold they have.
Explosives: The enemy team deals additional magic damage to structures for 3 minutes.
Only 1 Traitor Item can be purchased every 5 minutes.
For the enemy team, you would get an announcement like: "An enemy informant has granted you vision!"
Much like the surrender vote, you can vote to declare someone is a traitor. This vote can be made 10 minutes into the game and every 5 minutes after that. The vote must be 3 out of the 4 not being voted on. That player loses vision of allied champions and can be targeted by them. It also disable's the traitor items in the shop for that player if the guess is correct.
Killing the Traitor grants global gold to your team and a temporary global buff. A killed traitor will then officially switch sides and respawn on the opposing team.
Falsely accusing an ally champion and slaying them grants the same global gold the the enemy team and the the global buff.
This buff and gold should be large enough to warrant not wanting to guess wrong.
The enemy team would also receive announcements on traitor votes, for example: "The enemy team has declared a traitor!"
A successful guess will also prompt this message in the chat box after the champion has been slain: (Champion) has joined your team/the enemy team!