The "Recommended Items" that are displayed at the top of the store's UI should allow for a replacement custom build.
This will reduce the time spent looking for items in matches. allow for more income for Riot if made like extra rune pages. allow for adjustments out side of a match and still keep the game balanced and fun.
here are a few ideas for handling the RP costs if you follow through 1 custom build 975RP and that would be select able just like rune pages from drop down menu before match starts

Example

-Recommended
-Custom 1
-Custom 2

also allowing for them to be named for the champion or role that build would be for

you could make money with this idea don't just ignore it like some of the rest RIOT