Crossover like between different animes, or just pairings within one anime? I haven't thought so much about Ginga pairings (since I expend all my energy on shipping Ymir x Christa from Attack on Titan lol) but if I were to ship Ginga pairings, I'd probably go with Weed x Jerome and Cross x Lydia. But Riki x Fuji and Cross x Ben are also cute canon pairings.