So I'll start this off with a simple explanation;
Champion specific quick-cast is when I set quick-cast options for when I play a specific champion, and they DON'T carry over to the next champion I play, but next time I play the first champion the abilities are already set up with quick-cast.
**For example:** _I'm playing Nautilus and I want to quick-cast my Ultimate and nothing else. So I set that up. Next game I'm playing Brand, and I don't want my Ult on quick-cast. I have to remember to open the menu and change the options every game._
**Don't get me wrong, it's not that big of a deal and its habitual at this point, but I feel it's something that could make play a little smoother.**
All you'd need is a few small files (one for each champ) on the client side to read off which abilities are quick-cast.
I don't know how many times I've gone from playing a champion I don't want to quick-cast my ult on (Lets say Jhin), then next game I've played someone I really want to have it on quick-cast (Zilean). If I forgot to turn the ability back on quick-cast then I'd be standing there trying to ult and wondering why nothing happens, potentially losing a team fight or what have you.
Not game breaking by any means, its just something I'm amazed hasn't become a thing yet.
This also carries over for items as well. Imagine one game you're playing someone like Akali who you'd build a gunblade on. You put the item into your second slot because thats the one that's easiest for you to press. You figure it'd be easier and faster to quick-cast it than to try and press the button then click on the enemy, so you go ahead and set your second item slot to quick-cast. Good on you for making a decision to try and improve your play. The next game you end up playing a support so you do the right thing and buy a sightstone, and place it into your second item slot because you're most used to using that key for actives. Your team is trying to secure an objective and you go to place a ward over the wall, but low and behold its still on quickcast and you place it in front of the wall. _(You then all die to a blast cone gank by the enemy team because you had no vision. Not necessarily part of the example but entirely possible)_.
This is just the kind of thing that's make the game a little nicer to play, as I don't have to worry about my keybindings being setup for my champ every game.