I think it would be fun if your minion zoo was similar to your plushie or beanbag collection? I would love to get medals for submitting a certain amount of minions to your zoo. However I think this would cause the addition of minions to your zoo to be permanent, but if enough of us were okay with that, I think it would be nice to add some sort of reward to collecting rare minions.
Staff have said that there wouldn't be a permanent minion collection, because minions are technically sentient and are more designed as companions to your pet.
The way the minion zoo is designed right now is better.
I know Staff doesn't want to make them permanent but I do love the idea of any achievements based around the zoo.
Like 'Have [x] amount of unique minions in your zoo' and/or 'Have all food-themed minions in your zoo'. I think it would be similar to Wardrobe achievements which aren't permanent either. I'm very biased because I just LOVE an achievement.
It's been suggested in the Achievements section of this forum to have rewards/forum titles and stuff for collecting different sets of minions and things like that.
I agree, I think it would be fine to have achievements for the minion zoo with the way it is now Since there are achievements for having certain items in your wardrobe or inventory, and that's not permanent either.