That’s an interesting idea.
I initially considered using a combination of calc and vw unit (see this article) to make the font scale gradually according to viewport size while keeping coherent min and max heading sizes.
The main cons of this approach is that we would loose the use of the @font-size-hX bootstrap variables.
If we want to stay compliant with bootstrap 3, one option is to define 3 heading size scales for the small/medium/large media sizes that bootstrap 3 support.