I couldn't think of anything better to call this. :/

Maybe missions could be automatically completed after a certain amount of time? So, if a mission is over, but the player doesn't come and complete it after, say 24 hours, the mission would be automatically completed. Then when the player logs back on again they would get a message saying "number missions were automatically completed. Your rewards were:" and then lists the rewards you got.

Because once I was looking for a good mate for one of my pets, and I found the perfect one. But when I tried to breed with her, she was on a mission. No problem, just wait until the user logs back on. Then I look at the user profile and find he hasn't been on for 100+ days, so the pet is probably stuck on this mission forever. Sigh ...

I like this idea but more for it's common use(it's kinda rare for mission pets to end up like that). it's a pain accepting and retrieving the mission boxes.

a good idea indeed.
those people leaving their pets on a mission and then quitting the site forever... ugh.

however, I'd suggest that a significant amount of time should pass before auto-completion.
like, a week or even a month. so people won't get lazy like "I should finish my missions oh nvm they will be finished automatically soon".

another option is to make the reward for the auto-completed missions smaller.
I mean, the same way with the hatched eggs, they hatch by theirselves if not hatched by their owner, but they don't get bonus stats. maybe cut the gold reward by 25-50%.

_{(sorry for my English, hope I didn't make too many mistakes)}

I support! :D