Comics Alliance has gotten a motherload of artwork for the scrapped "TMNT: The Next Mutation" 4th live-action film from the 90s!
Some great stuff in there, such as a look at presumably alternate universe versions of April and Casey, a thorough look at Kirby's many faces, Super Shredder and John Woo-esque Foot Soldier sketches, and even some villains named in honor of the Mirage crew, such as (Jim) Lawson, (Eric) Talbot and (Michael) Bugman (Dooney).
In case you haven't yet, you can also read my older research article, The 4th Live-Action TMNT Movie that Never Was, for further information and artwork involving the scrapped project.
Neat stuff all around.