This day and age? A case! haha Not perfect, though. Cases are always extra bulk, no matter how thin they are (and if they're too thin, they don't really serve their purpose).
A box that you connect via USB to your phone and you then turn it into a PC. Like DeX, but that you can use your own cable to connect the phone or tablet.
With the amount of times people drop their phones these days, a super thick screen protector and a case would be the 'must have' accessory. Especially when you look at devices like the Galaxy S8 with glass everywhere.
Another "perfect accessory" would be a thick drop-resistant case with a 10000mAh power bank built in. This is ESPECIALLY important for power-hungry devices (such as my Alcatel Idol X+) that drain 1% each minute when running intensive apps and games. Such a case would also have to feature a cooling fan to keep the phone as cold as possible.