I'd like to see the RISC OS Pi team work on making RISC OS compatible with Berryboot.
For one thing, it would have to be modified to be able to launch the RISC OS kernel after a running Linux kernel. This may require some sandboxing to essentially run RISC OS on top of a very minimal Linux setup.
Secondly, it would have to support a RISC OS "Filecore" filesystem as a file on a Linux filesystem. This would be mandatory to work with BerryBoot's current filesystem.
Alternatively, someone could fork BerryBoot to allow RISC OS Pi to be installed on its own fixed-size partition (separate from the BerryBoot one) and booted into as with NOOBS, perhaps by merging some NOOBS code into BerryBoot (including the RISC OS install process). The boot process in this case would be different to launching Linux distros, which loads them with a common kernel without a reboot. Instead, it would have to reboot into the different kernel as NOOBS does.