i know what they were going to do!
Iggy - Apparently could breathe fire, use his hair as a horn and charge at you, and one of his animations looks like he's balancing on a ball, despite the fact it was Lemmy who used that tactic in Super Mario Bros. 3. He also has an animation where he grabs his buttocks in pain and collapses on the ground. Erm.
Morton - His attacks include jumping, spitting fire, charging at you, and strangely enough, inflating himself.
Lemmy - The original boss of World 3. His attacks include crying, jumping, spinning and spraying fire; judging simply by the crying and the spinning, his attacks seem most related to the whole vibe thing that effects stage enemies.
Ludwig - He had ability to grab something (either Peach or a boulder to throw or slam or something among those lines), retreat into his shell and spin around, and get dizzy.
Roy - His attacks include jumping, breathing fire, charging at you, throwing a spiked ball, and retreating into his shell to spin about.
Wendy - The original boss of World 6, and shee had an ability to throw one of her bracelets at Peach. She also has an animation where she points upwards and strikes a pose, and another where she faces the screen and stands with her arms in the air; presumably both were for summoning an attack from something else.
Larry - Boss of World 7. His attacks include spinning at ultra fast speeds, spitting fire, retreating into his shell and spinning around, and body slamming the ground.
I wish they had kept the Koopalings, as they're honestly so much better looking than the crappy Wiggler and giant Magikoopa and what not.