If I could wish one not produced toy into existence, it would likely be the Mego Palomino play set pictured here. I had a strong affection for the Black Hole as a kid and it continues to this day.
Here are the Land of the Lost action figures you never got. Buried in the pages of the 1976 LJN catalog, these "Sound N Action" figures are very similiar to the Mattel Heroes in Action (of which LJN was knocking off that year) in that the figures are attached to a base. As cool and mindblowing as this is, I'm a bit upset that these never got made.
This ad from Viacom was put out in all the toy trades at the time enticing licensees that Thundarr was a good idea. I wish more of them took it seriously. Heck, I still want the unproduced Mego line.