Since I was about to go to sleep, here is the quick and short answer so you don’t have to wait 8 or more hours before I reply.
I will be adding a new font asset type which will “Dynamic OS” where it will behave like any other dynamic font asset in the Editor. However, unlike Dynamic font assets where the referenced source font file is included in the build, it will not with Dynamic OS font assets.
When the font asset is first loaded on the target platform, TMP will get / cache the path and names of all font files on the platform and then provided it finds a font file matching the one referenced by the dynamic font asset, will use that font file as it source.
Just like the other font asset types, you will be able to assign Dynamic OS font assets as local fallbacks to another font asset or as global fallback. The order in which they appear in the local or global fallback list will determine which one gets used first.
If the source font file referenced by a dynamic OS font asset is not available on the target platform, it will be skipped. This will enable users to define a list of dynamic OS font assets for multiple target platforms.
A list of font files present on most major platforms is available from those companies. Most of these fonts are also available for download so you can import them in Unity to create dynamic OS font assets using them.
Using actual font assets as opposed to just some name reference, will enable users to define their Generation Settings and even tweak face metrics as needed to make sure these work well with any other font assets. Furthermore, this will ensure from a quality assurance point of view that the font file used in the editor will be the same and look the same as the one(s) on the target platforms.
Multi Atlas texture support will be available for Dynamic OS font assets as well thus enabling them to potentially handle every single glyph contained in those system font files.
In terms of ETA, I am planning on introducing those in the next release cycle which will be version 1.6.x for Unity 2018.4, version 2.2.x for Unity 2019.4 and version 3.1.x for Unity 2020.x. These might not be in the first preview release but will follow shortly thereafter.