iOS 13 should enable USB-C storage and make Safari a desktop-class browser. That would be the bare minimum to close the gap! Dream scenario would include pointing device support, a home screen rethink, and system-wide multi-display support.
A preference to force every site into "desktop" mode, through a combination of user agent, and detection. You should almost never see the phone version of a website on an iPad, unless you're in a split view or something. Download management. You should be able to see a list of downloads, manage them like you can in Safari, and save downloads into arbitrary locations via the Files app. This is already partially true. Tabbed browsing should be seamless and on par with desktop Safari, including pinned tabs. I'd ask for browser extensions, but Safari on desktop sucks for those as well, so its not really a fair request from a parity perspective. Developer tools on par with desktop Safari would be great, too, but not strictly necessary to close the gap for most people.