As a heads up, Riot's standard customer service response is "post it to the forums." This has been the answer given to numerous individuals who have contacted Riot directly with ideas.
This is because Riot does not take crowd-sourced ideas typically (a few exceptions exist, most of them skins). You're honestly much better of posting an idea to the forums and seeing if it gets a lot of support than waiting to hear from Riot. The only possible exception might be if you are otherwise qualified for a job at the company, and feel like pitching an idea alongside a resume -- but that's a VERY long shot, because, as someone who has interviewed there, they've got a LOT going on and are unlikely to drop an in-development idea to pick up something new without EXTREMELY good reason.
They're also extremely precise with the sort of people they want to work with, so if you DO decide to apply, make sure it's not just on the basis of one good idea: TONS of people have good ideas, and Riot is interested in those who can not only have good ideas, but also thoroughly analyze the cost/benefit ratio of those ideas, iterate on them, and know when to discontinue work on them. Among other skills.