@Riot There should be More In-Game Events between Rival Champs.
In Game rivalry events are a really fun addition to the game, and make the game a little more interesting. Rengar v Kha is the only one that exists that gives you a buff (i think), and I think there should be plenty more. Here are some examples.
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. If Xerath wins, he gains an extra ultimate charge. If Azir wins, he gains an extra soilder charge.
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. If Renekton wins, his fury gain is doubled (i.e 10 fury per aa instead of 5), and his Fury bonus does more dmg. If Nasus wins, he gains 50 stacks for his Q.
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If Sivir wins, her ult speed boost is increased, and her spell shield mana refund is increased. If Cassiopea wins she gains 50 stacks on her passive.
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If Garen wins, there are 2 villans, darius and the one with the most kills. If Darius wins, his Noxian Might and Hemorrage damage are doubled.
There are so many more rivalries that I can not think of off the top of my head, but there is a lot of oppertunities you can take.
EDIT: I know these are unbalanced, I just came up with them off the top of my head, but the main point is there should be more in game events. Riot can balance this stuff out.
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the winner gets the~~ banner of command~~... i mean 
gains bonus 1 ms when rengar is near
does bonus 0.1 dmg to twitch
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wins he gets a stack towards his e.
wins he spawns an extra voidling.
Extra rank in Transform, bonuses increase significantly
Free Hex Core upgrade (or Perfect Hex Core instantly)
ultimate now also grants cc immunity
abilities and aas reduce target's magic resistance (seems sub par I know, couldn't think of something better)
kills/assists reduce ultimate's cd, now it also scales on HP
now he switches with shadows from w and r as many times as he wants provided he is in range
: If Quinn wins, the radius of her Heightened Senses (W) is increased and buffed with truesight; if Talon wins, his Shadow Assault (R) lasts 1 second longer and gains an additional blade.
If riven wins her the 20% on her ult becomes 22%. If singed wins he gets 15% tenacity.
wins, his speed boost trail he leaves is more dramatic and his R mushrooms look upgraded
wins, his R will double in size and the effect upon hitting a target will be much more explosive. If it kills a target, Veigar will be temporarily surrounded with dark energy
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For every stack of the chasing down Jinx buff (1 stack for each kill/assist on jinx)