Ideas
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"No gameplay differences between either of the playable characters"

Seems like a blown opportunity, as even minor differences can enrich gameplay
and offer additional replay value. Second character could jump differently, or
use weapons in a different way that give the players different strengths or
weaknesses. They could even serve as a form of skill level selection if this
were to be carried out fully.

Mario 3 mechanic of losing access to areas might be considered old-fashioned.
Could consider a Super Mario World-style method of progress based on reaching
certain levels in order to save ("mid-fortress" for example)

Like 2-player simultaneous play, as that's a hot item right now with New SMB Wii.

Real-time inventory use could be implemented using shoulder buttons to select
items and other unmapped controls to use or put away the selected item, rather
than requiring the game to be paused, interrupting the flow of gameplay.

Slide - why not when crouching on any sloped surface, like in Mario games?

Parrot - if jump engages and controls, what disengages?

Diving suit - This could change controls to thrust-vectoring as opposed to
ordinary swimming motion.

Treasures - more might be better. In Wario Land there are major treasures to be
collected in almost every level, hidden in secret areas. The treasures then add
up at the end of the game to contribute to the ending received.