Well of course none of the Koopalings developed unique personalities in the game or talked or did anything besides merely exist. How were they supposed to when Nintendo forgot about them for years and years to focus on Bowser Jr.?
Maybe that's why they created Bowser Jr. They didn't want to worry about developing 7 individual personalities, etc., so they lumped it into one generic character.
Basically, I think the Koopalings are gonna be the "placeholders" for each of the castles so we don't have to stare at Bowser Jr. drag Peach around the overworld map... then not be killed until the end of the game.
You kill whatever's in your way just for Mario to stop 2 feet to the right after the switch is hit or whatever, lean forward staring at Junior, and then have Junior be like "DUR CANT CATCH ME.. LOLZ GINGERBREAD MAN," hops out the castle, repeat. We don't wanna see him get away tons of times! We want him dead. So.... that's what the Koopalings are gonna be for. They're gonna take the place of the boss/Bowser Jr. duo in the sense that.... when you kill the boss.... you KILL the boss, he's dead. You've done it. He's not running away with Peach to the next castle. He's/She's 6 feet under, disintegrated, electrocuted, burned, drowned, stomped, whatever... and you feel accomplished!