Who is "Miyuki-senpai"? When Mutsumi and Serinuma are at the shrine in episode 12, Mutsumi asks Serinuma what she wished for. She replies that she wished:


for the Lord and Akane to live happily ever after
for Shion to come back to life
for Captain Levi to be more dere
for "Miyuki-senpai" to notice [someone]'s true feelings


The first two are from shows-within-the-show "Kachu Rabu" and "Mirage Saga". The third one is Attack on Titan (which has been referenced weirdly often). What's the fourth one from?
Miyuki-senpai is from Daiya no Ace. The anime is centered around Sawamura (MC) and Miyuki's relationships as pitcher-catcher (the author said so). Sawamura wants Miyuki to acknowledge him as a capable pitcher, yet his rival on the same team, Furuya, as the ace, made Miyuki pay attention to him more. Sawamura struggled to make the best battery with Miyuki, hence 'notice me senpai'.