The number one thing Google could do to improve v8, IMO, is to give it an easy to use cross platform build system. CMake for example. V8 has repeatedly changed build systems. From SCons to Gyp and now to GN. All of these systems have been fraught with errors. Just try building the latest v8 on OSX, Windows and Linux. The SCons system was actually the most reliable. It may not have had all the fancy features of the newer build systems but it was much easier to reliably reproduce the builds.
The second biggest improvement would be to simplify the API. Using Contexts, Isolates, HandleScopes, etc. is complicated and causes the code to crash in unpredictable ways when you get it wrong. The API was built with Chrome's internals in mind and this shows.