I'm assuming Big Sur on ARM will be virtualizing ARM, not emulating Intel, meaning a VM will need to run an ARM Linux distro.
In the short term I expect that to be problematic. First party packages in the distro's package manager will be fine, but I expect it might be hard to find some third-party software compiled for ARM Linux. And any Docker containers in the Linux VM will need to use multi-arch or ARM Docker images.
I don't think it'll be all that bad: one advantage of Raspberry Pis having been out for several years is that there's already been some demand for ARM and ARM64 builds of software.
Closed source, commercial software might be a bit more of a crapshoot, of course, but this should provide a fair bit of impetus.
Actually, the rPi foundation have been extremely lazy about ARM64 support. The Pi 3 and newer support it (and I run 64bit Arch on mine), but Raspbian does not. I'd wager 95% of Pi users are stuck with 32-bit ARM software, as you have to go out of your way for ARM64 support.
If I am counting correctly, nixpkgs (NixOS) Hydra currently builds 22369 packages successfully on AArch64. IIRC also 98% of Debian unstable builds on AArch64.
So even though the rPi foundation may be slow (I don't know), the larger community has been working hard on making software build on AArch64.
To be fair, Raspbian is meant to support first-time users. They have explicitly said that they want to produce something that works for the widest range of hardware.
In the short term I expect that to be problematic. First party packages in the distro's package manager will be fine, but I expect it might be hard to find some third-party software compiled for ARM Linux. And any Docker containers in the Linux VM will need to use multi-arch or ARM Docker images.