The Return of the Egg

[Havard]

Was this just a plan (and nothing more than a plan) by Bruce Heard or were the adventures written and eventually not published?

The Return of the Egg adventures from the 1990s were just plans from Bruce Heard and TSR. They were abandoned when Mystara was moved to AD&D, but they might have been revived if the Mystara AD&D line had been more successful and if TSR had not declared bankrupcy a few years later. 

With WotC's acquisition of TSR, Peter Adkison made arrangements that eventually allowed Dave Arneson and Dustin Clingman to publish the Dave Arneson's Blackmoor product line for the d20 system (and later 4E), but the Return of the Egg plans would have been outside of what ZGG and partners would have been allowed to produce even if they had wanted to. 

-Havard