I would like that too. Not just the good ideas from RISC OS (IMO that's drag/drop filer integration to applications, appdirs, good font rendering), but also Amiga OS (public screens, assigns, datatypes) and BeOS/Haiku (media server, pervasive multithreading).
Well, you can often drop files on the main window of applications, and the open / save file browser is somewhat nicer than having the application window, the drop target window, and a desktop file browser window all open and sharing space when you really want to drag from one, and drop into the other. It was kinda neat, but I learned quite early on that you could usually drop documents on the target application in the task area below the desktop background. Not that the Acorn guide indicated that "feature".
!App (I think is what you're meaning) is pretty much still in existence on Macs. (*.app directories) I do miss the idea that Acorn curated the 4 letter application and 4 letter document type codes, but I also miss BeOS/Haiku background MIME type recognisers, and using extended attributes (or "alternate data streams" in Windows terminology) to remember what you've already scanned. In Windows, a PE is able to store an entire file structure so it was designed so it could do that. (it could, but it isn't used that way) I think Flatpak and docker are close on Linux, and BSD has Jails, which kinda work like that, or like a chroot on steroids? (they're good, but I've not used them much) Depending on your point of view.
Amiga public screens, on modern hardware, I don't think would be significantly different from multiple workspaces. (or desktops, in Windows terms, because even they are getting in on that now) Assigns, with a Posix root would just be a mount, or softlink. No? You can "assign", and "subst" in Windows still, but that's where the Amiga got it from. (Microsoft sub-contracted considerably on Amiga DOS and the original Amiga Basic, from what I understand) DOS drive letters need to die though, and you wouldn't run out of letters with the Amiga system. (not dissimilar to the content of Linux / BSD /dev/ directory dh0: would be /dev/sda0 in Linux, etc.)
Win32 doesn't need DOS drive letters. They're a "compatibility feature", but it's compatibility which needs to be deprecated, IMHO. You are supposed to program against the Windows Object Namespace, because then you support virtual folders like Fonts, and Control Panel and virtual views of .zip and .cab files, to comply with the old NT / 9x style guides, but nobody does. That's the \?\device\harddisk\0\1 being most peoples C: drive on XP thing, but if you've never seen it, I'm not surprised... Again, you can "link" complex object paths to more simple ones, and essentially that's what C: is, it's \?\dos-device\C, or something, in the namespace. (because that's how Windows "mounts" devices) That's what the NT versions of "subst" and "net use" do too... But you could equally "mklink" it yourself, you just can't "cd" to a namespace object, just as you can't "cd" to a UNC... Only browse them with Explorer.exe, and that tends to change the address bar to a known DOS style path when ever it can. Maybe PowerShell can? IDK, but I doubt it, they didn't extend cmd.exe in a sensible Win32-centric way, they started again from a .Net framework-centric way.
DataTypes... Yes. Admittedly most applications have file filters for importing comparable alien data, but the fact that it was a core part of the OS is the key. I think the flaw was that the IFF format was never "standard" enough is why it didn't really take off. Did you know QuickTime .MOV files and Windows .AVI are also IFF animation / movie formats, as is .WAV?
The other thing I miss from BeOS is the tagging of everything, not just MIME content in extended attributes, so you can search for every file by any of those tags, without there being a separate, obscure database which isn't open and transparent, either to the user, or to application developers to extend the system. (Spotlight / WDS / Cortana / even Zeitgeist) The fact that your eMail client could just be the file browser, a collection of monitored folders and a program that can read and write .eml files was just stunning simplicity. It would only really take freedesktop.org to include it as part of their standard for it to be taken up, I feel.