What exactly are you using your devices for? Since you asked, I would probably get rid of the ipad air and macbook air and get an iPad Pro with the pencil and keyboard cover. It has a big beautiful display that's great for watching movies, reading articles, playing games, using two applications at once, etc. You already have a well spec'd iMac (it sounds like at least), so you probably don't need two powerful macOS machines. And if it means anything, rumors have been pointing towards major software updates to the iPads with iOS 11. So maybe that ipad pro becomes something even better when the update arrives..
The imacs are getting refreshed sometime around march. Might be after march though. From what I have read, rumors point towards chip upgrades, storage upgrades,gpu upgrades, and usb-c. I doubt it will be anything major like the MBP, but maybe worth a look depending on which 2015 imac you are using. Personally I wouldn't get a 2016 mbp, wait until the 2017 model arrives with better battery life and bug fixes. By then we should see more monitors and USB-C devices that aren't so overpriced.
