With regard to Zinc, this all sounds reasonable. The only thing that isn’t explicitly spelled out here is support for fine-grained builds. Folks using Zinc via Pants (and Bazel, presumably) are encouraged to create lots of well-defined, small modules, and to use “strict” (ie, only directly declared) dependencies.
This allows for better cache hit rates against distributed build caches, but requires that Zinc support low overhead per invoke to avoid the actual compilation of ~2000 source modules being dwarfed by the startup overhead.
Finally, achieving incremental compile within jars would be a massive boost as well: ie, compiling directly to output jars, and then supporting incremental compile that consumes the slightly modified jars. The JVM hates classpaths containing loose classes in directories (anecdotally, jarring all inputs improved cold-build performance by 4x in one case due to drastically fewer syscalls and less IO), and this definitely affects Zinc when lots of modules are in play.