When TMNT hit movie screens last year, it was the green guys' fourth big-screen adventure, with a 14 year gap since the last one. You may or may not be aware that a fourth adventure almost came much sooner, in the form of a live action movie.
Obviously that movie never happened, but it seems like production got further along that I ever expected it did. I recently purchased a 1996 Playmates toy catalog where to my surprise I found this page teasing this movie and the toys that Playmates intended to tie into it.
Details of this proposed movie aren't well documented, but it is known that it was to involve a 5th Turtle, who would have been named Kirby (after Jack Kirby) - Concept sketches of Kirby can be seen in the front and back cover of Kevin Eastman's Artobiography book. This page uses Michael Dooney art created for the TMNT magazine (Fall '92 issue) with a Turtle-in-shadow also featured (I believe the shadow was created using a piece of Leonardo art, also by Michael Dooney, that was used as a pinup poster in the TMNT magazine). The Turtle-in-shadow would presumably be Kirby.
The only plot hints given on this page, which don't include any talk of a 5th Turtle, are in the description of "a weird world of parallel dimensions and gothic proportions." Winter '96 is the given release date.