I came up with an idea that is just like the double XP boost but instead it doubles EVs gained from battling Pokemon. I just think it'd be cool because I know there are serious EV trainers out there like me so I thought it could help people train easier. These will last 30 minutes just like the double XP boosts do too. For example a Fearow would give you 4 Speed EV training points instead of 2 and a Spearow would give you 2 Speed EV training points instead of 1.
That's an interesting suggestion. I don't know whether that's actually possible but definitely a cool idea!
Also, adding onto what Demo and Fusion said, things like EV Boosts, Anklets/Bands/Bracelets, Vitamins, and Wings.