RP to IP conversion
Pretty straightforward idea. Just a simple in-client converter where you can swap RP for IP only (so Riot doesn't lose money). The conversion rate would be based on the 975 RP/7300 IP tier (6.46 IP per RP ratio, let's round up to 6.5), or if Riot wanted to be super awesome, something like 10-1. This would also be useful for purchasing those high-IP-priced runes.
Example: Say I want to buy a new champion and I have 500 RP and 1000 IP. Let's say I want Sion (1350 IP and 585 RP). Now I just go and convert some RP into IP. So it would take 54 RP (446 RP remaining) to convert into 351 additional IP needed to unlock him.