I'm assuming this just mean we get to double everything? Here's what I'd do
New Fighters: Bandana Dee, Issac, Maxwell, Grovyle, Dixie Kong (semi-clone of Diddy), Kamek
New Echoes: Galacta Knight (
![Meta Knight :ultmetaknight: :ultmetaknight:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
) Shadow (
![Sonic :ultsonic: :ultsonic:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
), , Impa (
![Sheik :ultsheik: :ultsheik:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
), Octoling (
![Inkling :ultinkling: :ultinkling:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
), Lily (Maxwell)
Bosses: Duon, Eggman, Rayquaza, Duma, some Golden Sun bad guy, Sigma, Metal Face, Master Core, Tabuu
Stages: Assuming that every stage not from Smash Wii U was made from scratch, that means that there are... 81 more stages to be made. That's a lot, too many to list. Let's just bring back every stage from the previous games, make two new stages for every series, and fill in the gaps with new stages from other series (like Tetris, Shovel Knight, Nintendo Badge Arcade, Exitebike, and Snipperclips).
All other resourses are poured into a new Subspace-style mode. If, somehow, any time is left, throw together a new version of Smash Run re-using a lot of assets from this new Subspace