Current images of Betelgeuse are already amazing, but I was wondering if the EHT could be able to make a significantly better image, given that Betelgeuse is pretty big and pretty bright?
Presumably, yes, EHT observations could improve on existing radio observations of Betelgeuse (e.g. recent ALMA images and comparatively ancient VLA images). Any observations would likely be targeted at known photospheric mm or super-mm emission from the star, mapping radius and temperature changes. The ALMA data showed the existence of a spot $\sim1000$ K hotter than neighboring regions, which would ideally teach us something about convection inside the star.
With a (current) resolution of a couple tens of microarcseconds at $\lambda\sim1.3$ mm, the EHT would indeed improve on existing interferometer results by orders of magnitude (which I believe, in the case of ALMA, are the highest-resolution images of Betelgeuse, period, at any wavelength).