In short its a theory based off this Amazon listing for the Smash Bundle. It shows 2 empty slots at the end, one is for Isabelle, one is for the last proper newcomer. In addition, based on the pattern in which characters are listed it implies 1 echo fighter on the other side, specifically between Palutena and Cloud.
The bundle is now available for pre-order at Best Buy. $140 ($112 w/ GCU). Includes the game, pro controller, and steelbook.
Edit: Also comes with the $10 reward.
~~Edit2: Some people are reporting that it is appearing as sold out. I currently still see it as available but it might not be for long.~~
Edit3: There is a 1 per customer restriction.
Edit4: Sold out. Feel free to keep trying.
Link for just the pro controller, no bundle, which is still in stock
As of 5:14PM EST 8/28 it is available on amazon on a bundle with special edition of the game
https://www.amazon.com/gp/product/B07GX4GWC2/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
It's available on Amazon today:
https://www.amazon.com/gp/product/B07GX4GWC2
NOV 5 - 2:25pm Eastern Time
I'm kind of interested in the Smash Bros Ultimate Special Edition. Is this what it is and what it comes with? https://www.amazon.com/Super-Smash-Bros-Ultimate-Special-Nintendo/dp/B07GX4GWC2
Super Smash Bros. Ultimate Special Edition - Nintendo Switch https://www.amazon.com/dp/B07GX4GWC2/ref=cm_sw_r_cp_api_Db.bCbMRF13RS
Back in stock
It's like 3 times as much https://www.amazon.com/dp/B07GX4GWC2/ref=cm_sw_r_cp_apa_i_WYUdCbGESPXG2