In summary:
The rumor about Amiibo MIGHT have been debunked. Amiibo might NOT unlock characters.
Someone unlocked Ness in 2 battles, but there is debate on if Zelda can be unlocked in 4 battles. Then, something very odd happened: he had to challenge against Bowser to unlock him, but failed the challenge, and he hasn't had any 'rematch' pop up. Using Samus doesn't seem to get anyone unlocked.
The user tried her on Classic mode and unlocked Inkling, but as he was writing it, the prompt popped up saying he could rematch against him.
It has already been confirmed earlier, but World of Light does unlock characters for use in other modes too.
But then the fun begins. Someone claimed that on livestream, someone unlocked Bowser through Squad Strike, which is very odd if you ask me, as TC said he unlocked Bayonetta. Someone also unlocked K. Rool by clearing Classic Mode as Link, and then beating the Classic Mode in 9.2 as the unlocked character also unlocked Ice Climbers. But the users haven't currently found out how to unlock Wolf or Ryu.
Beating Classic Mode with Ness unlocks Jigglypuff. And on the 82nd post is shocking, the guy says: 'Challenger's Approach apparently doesn't always reflect who you lost to - guy lost to Jigglypuff, went there, and is now fighting Pit.' So, someone lost to Jigglypuff, redone the Classic Mode, and is now fighting against Pit? This is very interesting. At this point, it's not clear whenever unlocking characters are random yet, but it was said the second source unlocked K. Rool and Jigglypuff in same way as first source.
PurpleMadness beating Classic with Kirby unlocked Pac-Man, and at a point he also unlocked Captain Falcon. Shocking revealation: beating with Falcon unlocked ZSS, so ship confirmed
There is still a lot of speculation on Cloud in World of Smash - they're claiming that Cloud is not in World of Smash. If this is true, this is going too far: sure, SE is stingy, but not allowing Cloud in the adventure mode is going beyond. I don't like this rumor at all...
Someone is claiming that you unlock him after clearing the Adventure Mode, though it's just all 100% rumor.
Beating Classic Mode with Pikachu unlocks Villager. TC saw how the method works and is trying to hurry to unlock characters. Someone else said Ice Climbers unlocked Simon. Then someone else claimed that Fox was used to unlock Captain Falcon. You can unlock Miis by just making them, lol.
On page 4 (for me, it's set to 50 comments/messages per page), there are new informations. You unlock Pit with 10 battles, Inkling through 12 battles or Samus Classic Mode. Beating Classic Mode with Mario unlocks Sonic (Oh my god) and Yoshi unlocks Lucario. Furry time?
Some agruement of which has no sense.
Once again, new information, in a new stream. Inkling unlocks Incineroar. Guy has seen the streamer unlock Ryu through about 50+ matches, but has no idea nor about this nor how did the streamer unlock Peach/Wii Fit Trainer.
A lot of new information surfaces again; Bayonetta can also be unlocked through Mario, as long as he unlocks Sonic first. She also unlocks Luigi. Little Mac can be unlocked through Sonic. Ryu, Ike, Puff and possibly Y. Link, WFT and Peach can be unlocked via VS. matches too, but the guy has no idea if it's true or how many matches involved. Last update, which is currently the latest one: Young Link unlocks Richter..