I have been lurking here for a while, reading about all the leaks and speculation. I decided to make an account as there is an aspect of this character pass that I don't see brought up often when discussing potential characters.
These five fighters were planned to be the last five characters for Ultimate. I'm sure there was internal discussion about doing additional DLC should this batch sell well, but that was never a guarantee. Now with these being the only characters and this potentially being the last Smash game Sakurai works on (there have been some rumblings about this, depending on how long it is until they start development of the next one), I think this may inform us of what to expect from the last fighter.
Now, I should note that the fifth fighter we are getting may not have been originally planned as the last character of the pass. There is a reasonable rumor that Hero was going to be the first character released but Joker was chosen to be revealed at the last minute for the VGA. So it's possible that someone like Terry was going to be the last character, but for one reason or another they were shifted forward. I've also heard that more DLC was decided following E3, so there may have been time to change the schedule with FP5 and a post pass character. But since we cannot know this I'm going to operate under the assumption that FP5 was always going to be the last Ultimate character.
What does that tell us about the nature of the character? Honestly, nothing. We don't know for sure what the thought process behind character inclusions is. However, I would posit that we can make educated guesses about it. Joker and Terry seem to be "Sakurai picks". They are relatively niche and without much linking them to Smash, but Sakurai is a huge personal fan of the games/characters. Hero is in a weird place where Sakurai likes the series but it is also a massive deal for being the grandaddy JRPG. Banjo, on the other hand, seems to be a pick made for the fan requests.
The sample size of DLC in the pass is so small that there is no pattern to be made out, but if I had to make a guess it would be that FP5 is going to be another "Sakurai pick". If it truly would be the last fighter that he ever put into Smash, why not make it a special pick. I think that means a "hype" character is unlikely and whoever it is will end up being viewed as niche. I really have no idea who it might be; Sakurai loves many games after all. But this is where my thoughts lead me currently.
I may be completely wrong about what kind of character would be selected for the last character, but I think the context of it being the last DLC for Ultimate should not be ignored.