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I feel like a lot of older Legendary skins have lost its Legendary status. When I started playing League, Legendary got everything new. Particle effects, voices, etc. Now you can get MOST of that with a 975 skin. We can use the latest skin, Heartseeker Ashe, as an example. New particle effects and a back animation. Some Legendary skins just have new voices. Here is some ideas on some things to bring the status of Legendary back to those skins.
Gentleman Cho'Gath - Had a difficult time thinking for particle effects, but maybe a top hat knock up for his Rupture (Q). For a back animation, making some tea or cleaning his monocle.
Demonblade Tryndamere - Overall, give it more of a "Demon" theme. Mocking Shout (W) has a demonic roar to it and some sort of demonic head appearing above enemies heads to indicate the slow instead of the chicken head. Spinning Slash (E) have some sort of fiery vortex around him when he spins. Back animation smashing his sword into the ground 3 times and having a roaring sound.
Nunu Bot - Blood Boil (W) should have a noise to fit the skin the sound of steam being released? Ice Blast (E) shoot out a junk ball of bolts/screws/springs/etc to hit the enemy. Absolute Zero (R) should have again, bolts/screws/springs/etc should circle around him during the channel. Backing animation being Nunu fixing up the mechanical Willump.
Corporate Mundo - I find it hard to work more into this champion. But maybe some extra quotes upon using abilities. Burning Agony (W) saying "Mundo can't be distracted!" Masochism (E) "Mundo must work harder!" Sadism (R) "Mundo works fast!". The quotes on W and E would have to have a 10 second or so "CD" on it so it isn't "spammed" (you'd be surprised what annoys people). Back animation looking through his briefcase for a file, when he finds it he holds it in the air and when he returns to base its missing from his hand. (Maybe a follow up animation to show a panic in Mundo if you hold still after recall?)
Some others that could use a tune up would be Annie in Wonderland and Alien Invader Dinger, but I hardly see those so I don't know their animations/changes all to well.