How about some sort of instrumental based off the little rhyme going around the sub? You know, the "you are my sunshine" one.
OMG someone should translate the second verse into bakasharkspeak and then we'll have an OP song!
Edit: Couldn't help myself. Something like this?
Edit 2: Formatting
You are my Senpai
You are my Senpai, my only senpai.
You give me dokis, when seas are grey.
You'll never notice me, but I still l-love you.
You're a baka and kawaii desu ne.
The other morning, as I was swimming,
I-It's not like I dreamed you held my fin,
And when I noticed, Senpai beside me,
I opened my jaw and went *chomp*.
You are my Senpai, my only senpai.
You give me dokis, when seas are grey.
You'll never notice me, but I still l-love you.
You're a baka and kawaii desu ne.
WIP verse, short on good ideas:
I've always watched you, you've made me happy,
If only you would notice me.
But I didn't write this for you or anything...
(Alternatively, remove the first two lines and make these the last two lines):
So you better not get the wrong idea.
But if I see you with another, Neither of you will see the light of the next day.
You are my Senpai, my only senpai.
You give me dokis, when seas are grey.
You'll never notice me, but I still l-love you.
You're a baka and kawaii desu ne.
You're a baka and kawaii desu ne.