Creating ISOs isn't something we plan to add, at least in the near term.
Opus lets you go inside of ISO files as if they were archives, provided ISO is enabled under Preferences / Zip & Other Archives / Archive and VFS Plugins. (If double-clicking them does something else, another program may be assigned as the default, but you can still select them in the folder tree if you don't want to change what double-click does, as long as Preferences / Folder Tree / Contents: ZIP files and other archives is on.)
Treating ISOs as archives is great if you want to quickly browse and extract some files, but won't work if you want to run an installer (since the installer won't be able to see any of the other files). For that kind of thing, mounting is still needed.
Mounting is best handled by dedicated tools, and there are great free ones which integrate perfectly with Opus, so we don't see any need to reinvent them. I use Virtual CloneDrive but there are lots of others. Their right-click context menus should work automatically, and you can also set up Opus toolbar buttons to mount and unmount things if you like, provided the tool has a command-line mode (most do). There are some example buttons in the Buttons & Toolbars section here.
Windows 8 and above have built-in ISO mounting support.
Windows 7 and above have built-in ISO burning, so that's taken care of by the OS. (Windows also still has the much older built-in support for building up a CD image on-the-fly, which Opus supports. Just drag files to a new CD or DVD.)