If Lily would have returned Snapes love, or Snape had loved someone else that returned his love, what do you think would have been different?
Snape would have been a lot more cheery and maybe a bit boastful.
But idk because it would ruin the whole book if lily got with snape.
I mean we need Harry and James.

Would he have joined Voldemort?
I very much doubt he ever would.
Snape is much more intelligent than that.

Would his cruelty and bullying still have been there?
Snape, i believe, was never really cruel or a bully.
He was just jealous of james because james got the girl and the award and snape got nothing.
It's perfectly understandable, and that's why snape earns my respect at the end of Deathly Hollows.

Snape as a father?
Very strict but i think he would still be sweet. Of course he would want a slytherin son/girl

Would he taken more care in his appearance?
His appereance suits his personality.
It's dark and mysterious with a bit of an edge.
But he might've.

Would he sing, would he dance?
No