• Add cross compilation for linux-ppc64le: Add PowerPC build to CI. #9589, see also the list;
• Add cross compilation for darwin-aarch64: darwin-aarch64 build in CI #20245;
• Enable tests under QEMU for linux-aarch64 on our linux-amd64 servers: Add tests for AArch64 version of ClickHouse to CI #15174 (most preparation is already done, definitely will work; the server will run under QEMU and the client will run natively);
• Don't allow distributed queries across servers with different CPU architecture: Don't allow distributed queries across servers with different CPU architecture. #22581;
• Add cross compilation for illumos-amd64, see Adds support for building on Solaris-derived systems #23746 and Update submodules for building on illumos #24144 (it depends if lld can generate binaries for Illumos or if some other cross-linker exists);
• Try Darling: Validate Darwin build with Darling. #15609 (there is high chance that it will fail but we have to try);

Further steps:

• Publish deb, rpm, tgz and Docker images for linux-aarch64 builds in releases;
• Put headers and blobs (libc-related .so, crt*.o) for every toolchain in git submodules. Leave only required blobs (it is around 10 MBs in size - ok for git); Experiment with minimized toolchain (2) #29974
• Make default linux-amd64 build to use sysroot similar to cross-compiling for hermetic/reproducible builds; Hermetic builds #30011
• Allow to disable all the remaining usages of dlopen (loading of CatBoost) and make static build with Musl; (we will later introduce our own dynamic loader for simple use cases to avoid usage of dlopen) Preparation to build with Musl #30248