Assuming everyone from Smash 4 is returning-which is assuming a lot, I know, but I need to start somewhere-here are some additions I would like to see, separated by series.
Super Mario Bros & spin-offs
I'd like to see Dixie Kong, especially after she made an appearance in the last Donkey Kong Country game. In my opinion her chances will increase further if another such game is announced close to Smash 5's release.
Another popular choice would be Waluigi, who I am surprised hasn't made the cut already. He's only in sports games if I recall correctly, but that shouldn't be a problem as a lot of those games-especially Mario Kart-are hugely popular.
And finally, there's Toad. I think he's less likely due to not really being known for fighting, and if they were going to include him they probably would have done it already. Furthermore his most recent portrayal, Captain Toad, is unable to jump, and I don't see how that would work in a Smash Bros. setting. They could of course change his moveset up in order for it to make sense, which Sakurai is known for doing. So we'll see.
Legend of Zelda
The character I most want to see, as well as the character I voted for in the Smash Poll a few years back, is Vaati. We don't have any handheld-exclusive characters yet, and he would be an excellent villain to add to the roster. In the Smash Poll I suggested that he could be able to turn people into stone, which would have an effect similar to the Pitfall. I know this one is unlikely, especially since he hasn't made an appearance in a game for years, but I can dream. xD
The more likely addition should be Impa, who like Waluigi I am surprised hasn't already been included. The only thing against her is that she didn't have much of a fighting role in Breath of Wild, leading me to speculate that if she's included her design will be based off her Hyrule Warriors appearance.
Metroid
As I've said elsewhere, they'll either add Ridley, which is unlikely at this point, or they won't add anyone at all. Metroid isn't really known for its supporting cast, so I don't expect anyone else to be included.
Fire Emblem
If Fire Emblem Warriors is any indication, they currently only really care about Shadow Dragon, Awakening, and Fates. Thus my desire to see Lyn and/or Micaiah is likely not going to happen.
When considering just the aforementioned games, my top pick would be Caeda. But that's definitely not going to happen, so I'm thinking Tiki might have a chance. We already have a dragon in Corrin though, so her chances aren't as high as Ryoma from Fates, who unlike everyone else I mentioned, I would be surprised if he didn't get included.
Star Fox
First of all, bring Wolf back. He was the only 1st party veteran(bar Pichu) who didn't return in any capacity last time around. Even Roy came back. Even Dr. Mario came back. xD
I'd also like to see Krystal, but I highly doubt it at the point in time, especially considering Star Fox's popularity is in decline, not to mention the fact that she appears to have been erased from the series' current continuity.
Kirby
Similar to Metroid, I think we'll either be seeing a Waddle Dee or nothing, and a Waddle Dee doesn't seem too likely. It did get an amiibo though, so I suppose it's possible.
Pokémon
Very hard to say, given all the options. Probably Decidueye based off his appearance in Pokkén Tournament, but I can see one of the Ultra Beasts as well.
Animal Crossing
I used to think Tom Nook would be the most likely newcomer, but now I think it'll be Isabelle due to her appearance in Mario Kart 8. But like the Waddle Dee, I think she's unlikely.
Xenoblade
Probably Cross or Elma from Xenoblade Chronicles X, and almost definitely Rex or Pyra from the upcoming(and marketable) Xenoblade Chronicles 2.
Retro Games
I don't know much about these older games, but I do know that I want to see the Ice Climbers return. I mentioned Wolf as being the only one who didn't return in "any capacity" because Nana and Popo were programmed to be in the last game, but were scrapped due to problems on the 3DS version. That will no longer be an issue this time around, so I expect they'll be coming back.
New Series
The Inklings. Need I say more? ^_^
Third Party
Snake would be nice, but don't hold your breath with Konami. Tails and/or Knuckles would likewise be nice, but I don't expect we'll get multiple characters from a single 3rd Party series. Lloyd Irving seems to be a fan favourite from the Tales series, and I wouldn't mind seeing him included either. A Rabbid and/or Rayman also seem very likely thanks to Nintendo's new relationship with Ubisoft. And I'm pretty sure I'm forgetting someone, so I may come back later to edit him or her in. xD
However my #1 choice for a Third Party character is a former Second Party character, Banjo-Kazooie. Microsoft has even expressed interest in giving Nintendo the rights, though I still find it very unlikely unfortunately. What's more likely is Yooka-Laylee, who I would also be fine with.
Wow...I didn't think I would end up writing this much. {XD}