List of prototype or unreleased themes
Brickipedia (A.K.A. lego.wikia.com) list unreleased sets and themes in the form of an article and category of articles. It does mention CYBOTS and Europa in addition to Seatron, although there is no guarantee that the list is complete.

CYBOTS




  Contained action figures using both System and the first TECHNIC
  ball joint parts. Several prototype models were created but final
  versions were not made. This theme was made mainly to test the first
  LEGO ball joints, which would be later used three years later on the
  Slizer theme.


Europa




  Set in the 18th century. The most likely reason that the theme wasn't
  released is that it was too close to both Castle and Pirates, both
  very similar themes.


Seatron



  The theme would have been set on an alien planet, with underwater
  astronauts. Many new monorail parts and colours were made for this
  theme, but when Futuron's monorail failed to make money, development
  of Seatron stopped.


Unnamed prototypes

Here's a space theme using purple and trans neon green from 1989:



And another 1989 space prototype with a distinctive color scheme using both light and dark gray with trans-red elements (dark gray would have been a new color at the time):



Brickipedia has a similar list for prototype sets and themes.