Teeqoz on 09 September 2015
"Sleeper hit is a term used in the entertainment industry for a film that plays successfully for a long period and becomes a big success, despite having relatively little promotion or lacking a successful opening. It is also used in a similar sense for music releases."
Splatoon both had/has a lot of promotion and had a successful opening. Doesn't fit with term sleeper hit. Not really sure what will be 2015's sleeper. I'd say Rocket League 
but that doesn't fit either, cause it had a very successful launch.
