Picking up Players: NSMBW and NSMBU, unless done right is total nightmarish with this, especially how you can have your movement and jumps unintentionally lead to damage or death. If I suggested this, I'd prefer it to be more based on Rayman Origin's co-op system where if you hold up, you'd lift your arms up to make a platform, and you can stack other players on top of one another.
Also, player collision can be enabled with the "Player Collision" cheat in the Options menu. It's not picking up and tossing each other and even deliberately into traps, but it's the closest you'll get.
Picking up Shells: This'd be a better feature for certain maps, like 0' Brothers, but I don't really see it fitting anywhere else. Sure with SE having stuff from SMB3 I could understand, but SMW did shell tossing right. Holding up=toss it upwards, holding down=just drop the item without throwing it.
How I'd see this working in Mari0, the best bet would to include special variants of Koopas and Buzzy Beetles that would automatically be collected when you run into one, and then press Run to fire away.
Finish Star: I'd prefer this more as a block that beats the level when you strike it. It'd have a texture of a flashing star as seen in the SMB1 hack Super Mario Unlimited, which triggers his automatic walk into the castle/pipe/door/cave, finishing the level and giving out the time bonus.
As for the items, I really don't see much usage for them. We already have a flight-based form (Raccoon), Turtle already did the Ice Flower for his mod, and the Penguin Suit wouldn't work since there is no setting for slippery surfaces in Mari0.