Kindred, Syndra, Taliyah, Elise, Ryze, Jhin, Vayne, Cassiopeia, Kalista, Azir, Riven, Bard.... These are to play at a high level.
Kindred fundamentally teaches you the jg, how to play the strategy of jg due to their reliance on kiting, the nature of their passive marks (teaches you how to predict and manage camps, when to invade, how to invade.. the decisions leading up to it)
Syndra is highly reliant on positioning and timing, as well as micromanaging her spheres.. to a larger extent than Orianna in most aspects. Also, her ability to cast while moving is something very few people, if any, have completely mastered.... Theoretically she should be able to dodge every skillshot while landing her skillshots... and always be moving smartly, but most people will be unable to do this beyond kiting whilst running away.
Taliyah (and Ryze and TF tbh but to a lesser/different extent :/) particularly in regards to her ult... It may be game changing, can make or break games.. learning how and when to use it is key, She has no damage on it, and all her abilities are also skillshots soooo, yh there's also that.. The worked ground nature of her Q and passive means you need to learn how to position and think about how, where and why you use the Q.
Elise... is just difficult cuz she has 7 spells, transforming champs that have their kits change (nidalee and jayce) are difficult for this reason... Elise to a larger extent because her kit is more complicated imo.
Ryze is all about how you chain your abilities (Leblanc too to an extent...tho she's more binary usually.. wish they'd revert the revert)
Jhin, jhin is a beautiful champ who's currently overpowered xD... but yh, he's fundamentally different from ADC's in that his auto attack system is much weirder... requires you to pay attention to how you use them, he's also skillshot reliant
Cassio, much like Syndra, is all about positioning... Functionally acts like a marksman tbh. Managing the poison, moving and avoiding damage all whilst keeping up damage efficiency via the E
Azir is hard due to micromanaging his soldiers and their autos while also keeping in mind you are the champ too so need to move and avoid damage
Vayne requires mechanical skill like most adcs, however she's much closer ranged with no real skillshots, so is positioning reliant moreso than others... tho she does have her mobility and invis to help :/
Riven is reliant on animation cancelling, a fundamental technique to learn
Bard, Bard is hard due to his Q being reliant on other factors to stun, his E is about taking a magical journey and his ult is a potentially troll ability... alongside it being hard to land due to its timing.
but theres one thing to it: after they hit everything you end up dead or with really low hp and are hard to stop when fed bacause theyre simply oneshoting squishies and burning through tanks
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needs high macro knowledge.
is very hard to play. Through your description,I would say 
Support:Thresh
are for me the type of champs you are looking for, Nida needs as a jungler a extremly good pathing at invading because if you are not agressive you fall behind, she's also is a champ with a high skill floor and you'll feed at first. And even when you are stomping on Nida you can easily get caught and did, she's not a powerhouse.
is a difficult champion even though i dont play him, good luck trying to win if the enemy has high mobility or whatever you classify pyke as (i dont mean assasin something like chaos bringer or immobile engagers cockblocker)