I just played a game at work, which is usually 30ms ping and far far more reliable than my home connection. Today it was between 600-800.
I am aware that a lot of people live in great places with reliable ping to the west coast, but I am very susceptible to latency fluctuation as I live in Nova Scotia.
Having a simple ping test on the PVP.net client would have a quick and reliable way for me to know whether it's worth playing a game of LoL or not before actually committing myself to a game. Considering how important a positive competitive environment is to Riot, it's a relatively simple feature that would prevent leaves or general poor performance that ends up ruining the game for everyone.
I'm aware there are latency tests available online, but you could run ping tests off a $200 dell desktop sitting in the corner of the office and give all your players a dedicated and accurate indication of the ping they'd expect to get from your servers.
Maybe attach it to the "Play" button so if you have a ping of > 200 (arbitrary large ping) it pops up a warning saying, "DANGER: You have a stupid amount of ping and will not enjoy this game. At the very least you should pick a champion not dependent on being agile." That way it's not super spammable.
Anything warning me of my impending doom would be great