SOME NEMESIS DRAFT IDEAS [ WORK IN PROGRESS ]
Just some random thoughts you can up vote this post if you liked some of the ideas. you can also leave some of your ideas here :)
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Change summoner spells for the enemy - would be fun to give the enemy smite and cleanse but idk how every one will abuse that.
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All chat in champ select - dumb but funny because I cant see why it will be a bad thing
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Some of the players will switch character and build in mid game for the total madness (CD shared and the switch will happen only on your own team.)
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choose skin for the enemy - useless but funny (only skins the enemy own)
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choose one item for enemy to buy before they can buy anything else.
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Show enemy summoner names instead of summoner 1,2,3,4,5 so you can pick for them the lowest win rate champ.
with 
first item.