Heimerdinger, Master Yi, Twitch, Karthus, and Nasus don't have modernized kits. Their kits have been absolute terrors to balance. Karthus is currently not doing good at all. His kit has aged horribly. Nasus is a press Q to win after playing farm simulator for the entire game - which is again a very bad kit design. He's virtually unstoppable once he's at late game. Twitch had to be nerfed repeatedly several years back because of his kit, as did Heimerdinger, who needed a more substantial kit update last year because his kit was so busted against melee. And then there's Yi, who's been hell on wheels. We don't need any more updates like this because a VU alone doesn't fix some inherent problems in older kits.
Sivir, Tristana, and Gangplank have actually done well after their updates though. Except Sivir was a VU, and GP and Tristana were minor VGUs.
Also, you mention Alistar and Xin Zhao, but BOTH have had their kits updated substantially over the past 2 years, and Alistar had his model exterior updated. Alistar just needs a new VO, redone animations package, and more polish to his kit, and he'll be good. Xin needs everything visual and audio updated, and even then, his kit needs more refinement if it's to stay like it is.
I do not under any circumstance believe Morgana is "perfect" for this kind of update. Her kit needs major elements fixed, and it hasn't been touched or modernized, except to remove the spell vamp since Riot wholesale removed the stat from the game. Her kit encourages you to do shit from afar, yet her ult screams for you to be in melee range. This is a contradictory flow to her kit, and it needs to be streamlined so you don't have to suddenly try to change your attack patterns when you have your ultimate.
and 
who both gained a full VGU,but who were mostly just modernized in terms of abilities and personality.