Feel free to expand on it, but please only do so if you were acutally active in the community at the time.
Most people didn't see an Animal Crossing character as possible, because Sakurai had previously said it was too peaceful a series. When the topic came up, "Animal Crosser" as he was called, had some demand, but was overshadowed by Tom Nook/KK/Resetti/Isabelle requests. Overall, I'd say Nook was the most popular, with Resetti right behind, then KK, Isabelle, then Villager.
Common but Pessimistic Guess
Nearly everyone thought Mega Man was a good choice. He was a common spot on rosters and guesses. But people were pessimistic about Capcom allowing his use.
Near Impossible Guess
There's literally 2 records of her being guessed, and both were jokes. While most people seemed to assume that Wii Fit would get "A stage or something" the few times the subject was brought up, no one seriously thought she had a chance. She came out of nowhere.
Fairly Hard Guess
This board was pretty...obsessed with Waluigi and Daisy at the time (with about half of the supporters being trolls) and Toad and BJ were brought up. While Rosaima had a niche, she didn't become a common guess until after she was revealed to be in SM3DW. Then, she suddenly became a lot more popular. Most people assumed it was an Owain style fad. But it wasn't.
Easy Guess....In the West
A weird cultural divide. A lot of the English speaking community thought it was inevitable that there'd be at least one rep for Western audiences, and Little Mac was it, especially because his reboot gave him a lot more screen time. The "Smash Bros arena" blatantly being a Punch Out arena helped things too. Overall, the stage made him a shoo-in, essentially.
The JP audience...didn't see any of this, and was caught completely off guard.
He was also in Gematsu, which was taken semi-seriously then.
Very Hard Guess
At first, people thought a Gen 6 rep would be a shoo-in. But then, with no obvious choices presenting themselves, Lucario and Mewtwo acting as secondary posterboys for the gen, and Jigglypuff getting the Fairy type, the vast majority settled on new Pokemon stuff being rep'd by old Pokemon. However, some people did expect some sort of Kalos Trainer, as transformations were only revealed to be gone within the same direct that Greninja was revealed in. Then Greninja came and caught everyone completely off guard. While it was likely among the most commonly suggested Gen 6 mons, few thought they'd show off bias towards starters like that (in the sense that one starter would be alone). Turns out, they would.
"Pokemon from XY" was in Gematsu, which caused people to take it less seriously due to the vagueness of the statement, though it still had popularity...which grew after Mii Fighters.
Fairly Easy Guess
They were in Gematsu, which was a the height of it's popularity. Also...well, it was obvious they'd be in in some way. Thought they were shown as stage hazards, that really didn't hurt them too significantly in most peoples eyes, though it was points against.
Some people insisted they wouldn't be in, but that was mainly out of dislike for them. There was also some concern about putting them in fighting games, when Nintendo has shown that they're pretty protective of them before, because they are "real people."
But overall, they were a very likely, but unpopular choice.
Very Easy Guess
There was a leak that may or may not have been fake in the end, but the vast majority thought it was real. Also, she was in Gematsu, which was at the height of it's popularity. Sakurai...blatantly referenced her several times, she was removed from Pit's FS, and he generally just made it obvious. By the time she was revealed, few were surprised.
Like Miis, Pac was somewhat disliked by the community, making him get some people to say he had no chance out of dislike. Others thought that there were other, better choices. And some just thought he wasn't relevant anymore. A Namco employee helping with Smash even asked people to ask for a Tales character, which showed that not everyone at Namco wanted Pac.
However, he was in Gematsu at the height of it's popularity, and Pac was the company mascot that they always give a little special attention.
Very Hard Guess
Gematsu was more popular than ever, and it said Chrom. Plus Chrom was just the most obvious choice in general- why would Sakurai pick Lucina instead of Chrom, or even Robin (as seen below)? No one thought we'd get 4 FE reps either. She was also "an obvious costume" (Which turned out to be possibly true. She did originate as one, and seems to have the same model.) Most people also saw Robin as the "other" likely FE rep, after Chrom...once again, no one expected 4 FE reps. Furthermore, Lucina was seen as "Masked Marth" (as well as others like Tom Nook and Toad) during the part of Nintendo's E3 conference where they talked about Amiibos, so everyone saw her as disconfirmed at that point.
Somewhat Hard Guess
Like I said, all time high of Gematsu popularity, and it said Chrom. Even most people who didn't believe in Gematsu expected Chrom. No one expected 4 FE reps, so Chrom was it. However, people who saw Chrom as "just another swordsman ," as Sakurai would eventually say he was, commonly said Robin, as he would be a unique addition. Before Gematsu, he was a common "alternate" suggestion to the "obvious" Chrom.