Verrrrry interesting ! If the moon were replaced by something with a vastly greater mass but at the same distance, then ...
-- The period of its revolution around the Earth would be much shorter. That is, it would orbit the Earth in much less than 27.3 days. We might see it go through a complete set of phases in 2 weeks, or even 1 week.
-- The ocean tides would be much greater. Low tides would be much lower, and high tides would be much higher.
-- Sadly, the land tides, and the forces on the Earth's internal structure, would also be much greater. That means great increases in earthquake and volcanic activity.
-- The Earth and moon both revolve around their common center of mass. Under the current arrangement ... with the Earth having 80 times the mass of the Moon ... that point is inside the Earth, and it looks a lot like the Moon is orbiting a stationary Earth. When the new body arrives to replace the lightweight Moon, that point will be a lot closer to the new companion ... maybe even inside it. Then, it will look a lot like the monster is the stationary one, and the Earth is orbiting it. I actually don't believe that we would SEE that change, or feel it.
