I don't know if I am the only one that makes item sets for champions, but one thing that annoys me is the fact that I can not see the champions recommended build while making them. For me I usually use that and mix it with the way I play the champions. If this could happen it will help a lot.
For me, I will make a champion-specific item set when I find that the recommended is unhelpful to my playstyle. At that point, almost none of the items on the recommended page are useful to me and I build the set from the ground up completely based on how I play the champ.
I, personally, wish that the analogous items of the different modes would be added automatically. It's a pain to remember to add those items in addition to what I'm used to for SR.
Example: If I place the Bloodthirster 
on my custom set, then I wish it would also add the Sanguine Blade 
automatically, since that is the direct analogue of the Bloodthirster on Howling Abyss. Can't tell you how disorienting it was the first time I played ARAM and found half my items missing from my item set and having to figure out what I needed to buy to get similar stats/effects from this new map.
