I know most of this has already been mentioned but I'll through my hat in the ring anyway.
1.) Document Management System, we don't necessary need access to the entire file-system but we do need a better system than what is currently in place as it's really bad. I want to see a centralized DMS in which all local files are stored, including access to not only cloud services but FTP and NAS as well, with Open ID and LDAP support for secure NAS. This also means a single view in which apps utilize when accessing and saving files, the fact that apps currently manage their own documents is frankly absurd.
2.) Walled Garden, this needs to just go away. As of right now more than half of the apps I have installed in my iPad Pro don't support the resolution, instead their pixel doubled or zoomed in. Not only that but their using a different keyboard other than the default. This is because apps are basically islands in iOS, self managing entities that utilize API's to communicate with the OS in an extremely inefficient way. App developers have to manually create share profiles for their apps in order for them to communicate with other apps installed. Also with every single new feature that is added to iOS, example; dual app view, app developers have to program in these new features into their apps in order for their apps to utilize them. Unlike other mobile OS's like Android, in which when a new feature is added to the OS, every single app than supports it. For example when I enabled multi windowed support in Android 6.0 for my Pixel C, every app I had installed from that moment on just worked. The Walled Garden really is the root of all evil in iOS. We would also get features like multi-user support, without using a hack to do it like is the case with the educational iPADs, each user has to reinstall every single app because there is no other way to decipher which files belong to whom. Apps would no longer have to use API's for everything, the list of benefits defiantly out weigh the negatives by a factor of 10.
3.) Full multitasking, as in the ability to run apps in the background. This excuse of running multiple apps in the background causes faster battery drain needs to stop. Both my Blackberry Priv and Passport constantly have apps running in the background and they both last longer than my wife's iPhone, my Passport as much as an entire day more and I have a terminal running in the background constantly. The iPad Pro has 4GB of memory, I want the ability to run a terminal in the background, while I stream a film directly from OneDrive without having to first download the movie to my TV, while I convert a .avi to .mp4, while my son plays Modern Combat 5. Sounds like a lot but it really isn't, my Pixel C, Blackberry Priv and Blackberry Passport can all do this without issue, this is 2016, stuff like this should be second nature for any modern OS. To appease those who think running apps in the background has a huge impact on battery, iOS could utilize what Android has been doing for years now, have a simple user modifiable limiter, mine is set to 5.
4.) The ability to select your own default apps, this is a no brainier and the fact that this even has to be put on a list just show how much needs to be done to iOS before it can ever be thought of as a Pro system.
5.) Multi-user support with Open ID and LDAP support, this is a huge one for me and a lot of others who want to use their iPads in a Enterprise setting. This also ties in closely with a new document management system as after you login, you should than have access to your user directory on your firms NAS.
6.) Better external display support, actually I'm just going to say that iOS still needs support for this as in it's current form it's unusable. I want iOS's desktop to be extended, I want the aspect ratio and resolution supported and I want the ability to change the DPI to reflect that of a normal desktop UI.Though again, this has everything to do with that Walled Garden crap so we'll never have decent support for external displays until this goes away. Independent resolution is not a feature guys, it was a bi-product from apps having to basically fend for themselves, apps have just way to much responsibility in iOS, their practically OS's in themselves.
7.) Mouse support, yes mouse support. I completely agree with what Tim Cook said about noteBooks with touchscreen's being a failed idea do mostly to bad ergonomics, as in the user constantly has to reach up and over the keyboard to navigate the UI, ironically though the iPad Pro falls into these category. At least it does for me as the experience is identical to that of using a laptop with a touchscreen. Though unlike the notebook which has access to a trackpad, the iPad Pro has no such luxury. When using a keyboard with the iPad Pro, a mouse is a much needed add on for a lot of us. Yes, the iPad is a touch only device, however, the iPad Pro introduced a manufacturer built keyboard accessory and stylus, two input devices that greatly changed this paradigm, so adding support for a mouse shouldn't be a issue, if it is for some of you, than without being a hypocrite so should the other two.
Everything written here is nothing special as I'm using them daily on my Pixel C. Even external displays work like a champ, the desktop is extended so I have access to two monitors, aspect ratio and resolution are supported so no black bars or fuzzy image and I can modify the DPI. I even have profiles that are automatically triggered depending on the external source, TV, monitor and projector. I'm not saying buying a Pixel C is the answer either, especially when most of you hate Android anyway. However it's the reality that I'm faced with everytime I try to use my iPad Pro for productivity work, I'm constantly frustrated, especially when the device next to me has these capabilities. I don't mind having one device for work and the other for play but I would much rather just use One. Unfortunately that one device is quickly becoming the Pixel C, especially now that I'm dual booting Android and Arch Linux that is utilizing native GPU drivers from Nvidia. This is a big deal when I even have all of my CUDA apps and development stuff running, which means I can encode media files faster than most desktops can that are utilizing their CPU's to do it.
IOS just needs to be completely redone as it hasn't changed in any fundamental way since it's incarnation. Sure we've gotten more features, features that were frankly just playing catchup, however nothing ground breaking. We applaud Apple for changing the landscape of mobile, however since they now dominate it I expect their OS's to be the best on the market, I can say without a doubt that OSX is the best desktop OS, however iOS, well, it just needs work, a lot of it.