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.
Brickipedia have a similar list for prototype sets and themes.
