In the Iceberg Lab, just before Crash and co. teleport to the 10th Dimension, Dr Cortex says "Come now, as we explore a new dimension! It should have been two new dimensions, but we ran out of time." This is a reference to the development of Twinsanity, as they were originally planning to have six worlds, but because of the deadline, only four were finished.
There was also a cut level with its own music called "Gone a Bit Coco" and showed Dr Cortex running around inside Coco Bandicoot's imagination. The level was scrapped, and replaced with "Boiler Room Doom". The music didn't fit the atmosphere of the new level, and was scrapped too, however, one short clip of the music can be heard in the end credits, despite never appearing ingame.
CLIP OF LEVEL: http://www.youtube.com/watch?v=BG_cA3ayf...r_embedded
There was originally going to be a sequel to Crash Twinsanity titled Crash Twinsanity 2: Cortex Chaos. The plot was going to be Cortex holding a supervillain meeting and getting kicked out from being a villain after helping Crash during Twinsanity 1, and it would be up to cortex to get his revenge. Concept art shows cortex clones attacking a huge enemy a la Pikmin. Eventually, the game was scrapped.