On the homepage of the shop, it shows stuff like "Essential Items" and "Defensive Items" and "Offensive Items".
What I am proposing is that another section is added, called "Custom Items", where you can add items you want to get (that could be customized by champion, or possibly another drop-down on the select page as for runes and masteries).
This would make it easier to get certain items that aren't on the homepage, rather than searching around the shop for them.
For example, if I wanted to build AP Garen with Rabadon's (I know, that's not even worthy of being called a troll build, I just made it up, who would even run AP Garen?), I'd have to look in the shop for Rabadon's Deathcap. However, with a custom items section, I could set in the champion select custom items to "AP Garen" or whatever I named it, and then find Rabadon's easily on the homepage of the shop.

You can do this with Enigma's Item Changer (which is currently in "allowed, but updates might break it" territory). It would be nice to have it as an actual in-game option, though.

agreed and signed

Like the other guy said, we got Enigmas's Item Changer.
http://enigmablade.net/lol-item-changer/download.html

Are you suggesting something like a "Favs" button in the shop area? If so, take it one step further and have it automatically deduct gold to purchase the items in the favs list as the gold becomes available. That way you can concentrate more on team play rather than searching the shop for the items you want.

I had the same idea awhile back.