Idea 1: a hammer that tux can throw or attack with, like the super hammer from mario maker kinda... but only one at a time, either thrown or being held...
idea 2: the revenge in redmond cape... It lets you spin attack or in the air, it could be timed to act like mario world's spin jump... for a second... trying to use it on something that isn't vulnerable from the side will bounce you off harmlessly...
idea 3: a santa hat that just counts as an extra hit; getting hit will just revert you to bigtux, that's all... (get an egg while you're bigtux)
idea 4: A hyper fish that makes tux jump higher and run a bit faster; running into an enemy like this, if they visually don't have spikes, will actually bounce you back harmlessly instead of making you lost a powerup... however, it's only temporary, like the star...
Feel free to post more ideas yourself!