The problem with the recent legendaries is that they’re champions who are quite modern and new, which means they don’t have that special quality transformation when they get a legendary.
There’s a lot of champions that are old enough to feel outdated, but modern enough that they don’t really need a visual update any time soon. An example was Vayne, a champion who is quite old, but when she was given the legendary PROJECT skin, it created something special and unique with its new auto and ability animations. Others include Ahri with Star Guardian, Zed with Galaxy Slayer, Ashe with Project and Teemo with Omega Squad.
There’s a lot of candidates that fit this category like J4, Soraka and Nami. They’re old enough that a legendary skin on these would have so much better quality with newer animations.