Bowser's Castle is the final area in Mario & Luigi: Superstar Saga and Mario+Luigi: Superstar Saga + Bowser's Minions.
One Possible Change[]
Note: The following section is a fan-based idea involving an idea for a cutscene just before the final boss. Please disregard anything you read because this is fan fiction.
If the makers of Mario & Luigi: Superstar Saga were to add one additional cutscene before the final boss, it would involve Prince Peasley reminding the Mario Bros of the upcoming final battle with Bowletta/Cackletta.
Here is what the cutscene would look like:
(After defeating Fawful and entering the door, the Mario Bros. come across a big room with a spiked floor and a stairway leading up to a great big red door. Bigger than the other big red doors. Just as Mario and Luigi approach the door, Prince Peasley appears. The music fades out.)
Prince Peasley: Mario! Luigi!
Prince Peasley: I have secretly scouted the next room beyond the big door ahead of you. Bowletta is sitting upon Bowser's Throne within the Throne Room behind that door. It seems that she has been waiting all this time for you to come.
Prince Peasley: After overcoming many of the hard trials within this castle, it now all comes to this: The Real Final Battle with Cackletta.
Prince Peasley: You've come a long way, Mario Bros. But now, this it it. But before you go in there, I would think that it would be of vital importance for you to prepare yourselves for the last and final battle that lies behind that door.
Prince Peasley: If there is anything you should do before going in there, I would strongly recommend doing it right away. Because once you face off with Bowletta, there is no going back.
Prince Peasley: There is a save block and some item blocks here that will help you prepare for the battle ahead. But I would strongly recommend saving first.
(Luigi speaks to Prince Peasley)
Prince Peasley: I wish I could help you, but I am planning something very special for the main villain of this castle. What is it, you may ask? I can't say, for it would spoil the surprise. But know this: You should fall or triumph in battle, remember that the fates of both the Beanbean Kingdom and the Mushroom Kingdom rest squarely in your hands.
Prince Peasley: Now, I must be off to put the finishing touches. Mario, Luigi, from here on out, it's all up to the two of you now. We're all counting on you! Whenever you two are ready, head into the throne room to put an end to that fiend's evil ambitions once and for all! Good luck.