Here are my happy ending for each universe:
-U1: Umm... the Kais keep eliminating all the threats and U1 remains a peaceful place.
-U2: No need for an ending here.
-U3: King Piccolo and Bardock have a change of heart and become friends.
-U4: Buu uses his universe's dragonballs to transfer the souls of the Z fighters he absorbed into new bodies, so everyone is resurrected without Buu becoming weaker.
-U5: Unknown story as of yet.
-U6: Already got its happy ending.
-U7: Gast finds a way to split into the Namekians that fused into him.
-U8: King Cold has a change of heart (I don't think Ginyu is beyond redemption like Freeza and Coola) and becomes a benevolent ruler. Freeza/Cold remain bitter, but their 'father' keeps them under control.
-U9: Yamcha takes Androids 17/18 from U14 to his universe and helps them overcome their bitterness. The two Androids decide to stay there permanently. Eighteen and Yamcha become a couple.
-U10: Same as U1.
-U11: Already got its happy ending.
-U12: Nothing changes.
-U13: Raditz's plan succeeds.
-U14: With the Androids no longer around, people rebuild their civilization.
-U15: Unknown story as of yet.
-U16: Vegetto is somehow defused. I don't know whether this is a happy ending, but I'd like to see how those versions of Goku and Vegeta would act if they came back after so many years, moreover having both slept with Bulma while they were fused!
-U17: Cell is somewhat softened and shows some love to Cell Jrs. Also, it would be a good addition if he created a wife for himself.
-U18: Nothing changes.
-U19: The Heloites manage to save their universe from carbonite.
-U20: Broly somehow dies, and, in the afterlife, he finally rests in peace.