May be we can see higher resolution if we build an additional telescope.

More distant (longer baseline), not merely additional sensors.

Resolving power is proportional to the (virtual) aperture size, not the total sensor area (that gives more signal strength).

Given the observation period has been multiple years, does that virtual size include the orbit of the Earth? Or is there something that limits it to still being Earth-sized?

So put some telescopes in orbit around the Moon and Mars.

A guy on Reddit actually asked this in the AMA. While that would increase the resolution, it would also be extremely difficult. The algorithms used to combine the data from the dishes relies on the exact position of the dish being known at the time of measuring, to a precision of fractions of millimeters. It's already hard to do on the earth's surface, but imagine doing it with a sattelite zipping around the earth at 20K km/s, or the moon at >1 km/s around the earth, or Mars at 24K km/s around the sun.

Not to mention getting the data back down here. For the analysis of M87, there were multiple petabytes generated: they had to use good old sneakernet and ship hard drives.