So let's assume that Newt is a half-blood wizard. Graves comes from a long line of purebloods. Newt and Graves get together and the Graves family does not approve. Percival is suppose to marry someone respectable and carry on the family name and this magizoologist he is fooling around with is none of those things. So they make him choose between his family or Newt.

He chooses Newt.

Bonus for someone running into Percival and Newt months/years later and they are still happy and in love.

Nothing is really off limits for a fill. If you want to add mpreg that's fine. I think it would be great is someone ran into Percival and Newt and they had a kid or two.

Tl;dr - The Graves family makes Percival choose between them and Newt. Percival chooses Newt.