I can make a LoZ thread later so we can keep this related to FE's representation, but I'll give my take on LoZ newcomers here.
I feel like it would be nice to see a new LoZ character since there hasn't been a new one since Brawl, but it's not a major concern, and I mostly understand why we haven't received a newcomer since then (the only weird thing I can think of is Impa missing For's base roster).
If I got to add LoZ newcomers, Impa would be my first choice since she is a recurring character, and she also is relevant with her appearances in BotW, Candence of Hyrule, and AoC. I find it a little weird that she missed Smash For's base roster since SS was relevant at the time and she has appeared in multiple games, but I liked the choices for base games' newcomers, and they all made sense in the end. For Ultimate, the focus was on fan requests and newer characters, so she unfortunately didn't have a shot. For SSB7/FP2, I think she would be a great choice.
I think Skull Kid would be my second choice since he was a major antagonist of one game, appeared in OoT, Tp, and Candence of Hyrule, and is the most requested one. After that, I'd say Tingle since he has a spin-off series despite his hatred from the West.
For "one-offs" like Midna and Ghirahim (they're technically not one-offs since I'm counting Hyrule Warriors), I assume from a marketing perspective that it would just be more reasonable to just update the playable protagonist since the one-offs added who weren't the main protagonist were tied to other characters' move sets (Sheik as a part of Zelda's move set and Ganon(during OoT)/Lucina as Falcon/Marth clones). If that was the case, a Wolf Link/Midna tag team might have made sense in Brawl if it didn't have the same technical issues as the Diddy/Dixie one, but it still would give us another Link. If one of these characters ends up having Ridley, K. Rool, Banjo, and currently Waluigi and Geno (before the Mii costume returned) levels of demand, these characters could have a shot.
Related to FE representation, I'm not concerned if FE has more reps since SSB's roster is not primarily about giving each series proportional representation and showcasing just characters who have large legacies, otherwise we wouldn't have Ice Climbers, Lucas, a Metroid rep or two (while I'm fine with what I'll soon mention, Metroid does have more reps than DK, Kirby, and AC), possibly Ness, Banjo, and Jigglypuff, Falco or Wolf, Palutena, and most of the FE cast (it'd most likely just be Marth, Chrom, and mayyybe Ike).