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.