A few years back I worked with a partner ISV that sold a Unisys mainframe emulation product. At the time they had a pretty healthy business moving people off physical Unisys systems onto emulated ones - if memory serves, the emulator ran on Windows.
As far as I know, they're still around, and so is their market. So there's still enough of an installed base of applications running under at least one Unisys mainframe OS to support a business based on migrating applications to an emulator.
From a high level, OS 2200 is not that different from IBM's zOS and predecessors. You have batch and online processing, with a transaction monitor, etc. So people run back-office business apps: billing, customer management, inventory management. That sort of thing.
IIRC, the main interactive user interface mechanism is a block-mode terminal protocol that's not so terribly different from TN3270. There's a C compiler, so if there's something vaguely like BSD sockets available (and I bet there is), you could probably port the X11 client libraries - assuming that hasn't already been done - and display on an X server on a Linux or Windows box, to get GUI support.
OS 2200 is quite secure. It's a B1 system. Has a fair number of integration points with desktop OSes, too, thanks to things like Kerberos and CIFS support.