I have been planning ahead for a while now on what to do for macOS 10.14. So far, I have been able to get Snow Leopard running under Parallels and Fusion, though both don't support, 3D acceleration, for any version of macOS/Mac OS X. This means when I run MarbleBlast, there is just a black screen, there is sound as you can hear the music and also if you move the mouse around the black window, you hear the buttons being hovered over. Later on a demo starts to play, then crashes.
I tried to do this in both a High Sierra and Snow Leopard VM, (10.6 required an EFI patch to run as a VM to bypass server OS check). Both VMs displayed black screens. I would prefer using Snow Leopard as this also has support for PPC applications. This would mean that it could have more use as well as just for 32-bit support. Though the 3D acceleration is not supported so doesn't run most 3D stuff, just the OS.
I have an iMac 13,1 (Late 2012) and did manage to boot (not virtualise but actually boot) Snow Leopard onto this machine. I used Parallels to run the Installers, and installed it onto a USB Hard Disk. After replacing the mach_kernel for an Ivy Bridge version of it in the root of the USB disk, it could boot, but exactly the same graphics issues happened. Plus, there was no sound.
I tried this same method on a Mac Pro 3,1 (Early 2008) which came with 10.5 (Leopard, not SL). Now the graphics was changed from its original to a GTX 680. The installer DVD would not boot, because of graphics card not supported, so I had to installing using the way I did on the iMac, though kept the default kernel the same. There was sound, but again, black screen for MarbleBlast.
So there are graphics issues on all 3 methods I have tried, for Snow Leopard. VMs won't support 3D acceleration and neither does the 640M in my iMac, and neither does the GTX 680.
Keeping a small partition of High Sierra is probably the best thing to do as then I can run 32-bit apps, though no PPC support, you see it would be nice to have the best of all in one OS, which is Snow Leopard. I have seen someone running Mavericks under QEMU/KVM, on a Linux host, with 3D acceleration in the guest, but I am not sure how he did it and probably wouldn't perform as well than in Fusion/Parallels/Native Boot. What do you think is to be done?