.Net Core is not preinstalled, but there’s an installer step for that (more info: Weekly Virtual Machine / Stack updates - 2017.05.06 & https://github.com/bitrise-io/osx-box-bootstrap/pull/44)
That step is not maintained by us, please create an issue on the step’s github/tracker (you can find the link to that in the build log, in the footer of every step, as well as in the workflow editor, if you select the step → right side GitHub icon).
Regarding a stack with Visual Studio for Mac: at this point all I can say is that we’ll start preparing the new stack in the upcoming weeks. It’s at the top of our priority list. Unfortunately we can’t commit to a release date / ETA yet, as we’re yet to see if there are any significant tooling / step changes required. If everything goes smooth the stack should be available as a new stack option in the next couple of weeks, but we’ll see how much additional work will be required to update all the related steps/tools.
Once the new stack is available we’ll definitely post about that in Changelog, with the stack-updates tag.
In the meantime if you have any questions or comments please let us know!