If we could assume that most of the mm-wave emission from an ordinary star is photospheric, then the EHT could make a massive contribution to measuring the radii of stars.
At the moment, this fundamental property can only be measured for stars in short-period eclipsing binaries or for a small set of nearby stars and more distant giant stars using infrared interferometry.
The state of the art for the latter is the CHARA array, with an angular resolution of 200 microarcsec. The EHT can do 10 times better, opening up a thousand times more targets for angular radius measurements, that can now be combined with Gaia parallaxes to yield physical radii.
This would mean that we could properly investigate the mass-radius relationship in low-mass stars, establishing whether rapid rotation and/or magnetic fields make them bigger. This would also lead to better determinations of the properties of transiting exoplanets.
That much I know about, but I suspect there are other, rarer types of star that might be brought into reach and others could be studied with more precision. I imagine that following the radius evolution of pulsating variables like Mira would be easy - they have angular dianeters of $\sim 10$ milliarcsec. But the nearest Cepheids have radii of around 40 times the Sun at about 400 light years (e g. Polaris). This would have an angular diameter of 1 milliarcsec, so significant progress might be made here.
Another place where super resolution at mm wavelengths would be highly advantageous, is in the study of protoplanetary disks. The mm-wave observatory ALMA has already yielded some exquisite images of disks around nearby young stars at angular resolutions of tens of milliarcsec. These reveal the possible traces of rings and gaps marking the onset of planetary formation. Presumably, observations at a much finer scale could be used to test detailed hydrodynamic models.
Of course, I have no idea if any of the above is feasible in terms of source surface brightness!