Actually, we have two 3rd party items already!
The Boss Galaga and the S Flag are both items borrowed from Namco’s collection, rather than Nintendo’s. As for additional items from the third parties in Smash?
Sonic’s classic TV Monitors could make an appearance. The shield would be the most likely of these, as it is the most iconic. As for Megaman….
Eddie’s not a bad option! He could show up and throw items everywhere! Though that might be more AT material. Megaman has a ton of weapons in his series, but we still don’t know which ones will be used in his custom moves, so we can’t really say which ones would be good items! We don’t know what’s available….but hopefully we’ll know soon!