After both Marth and Ike being confirmed to return, after Robin and Lucina being revealed in one go and Roy being back as DLC, we've got plenty of FE characters from across the series' story.
I don't think we'll be getting another one. The series was already lucky to have 4 characters given Lucina's status as an alt-turned-clone, let alone 5.
This coming from a fan of the series and I honestly wouldn't mind if it had a 6th character.
If we're talking about FE sagas representation in Smash 4:
- Akaneia: Marth, music from FE1-3
- Jugdral: music from FE4-5
- Elibe: Roy, Lyn (AT), music from FE6-7
- Sacred Stones: music
- Tellius: Ike, music from FE9-10, trophies in the Wii U version
- Awakening: Robin, Lucina, Arena Ferox stage, music, trophies in the 3DS version
- Overall (based on recurring elements of the series): Castle Siege stage, Coliseum stage, Fire Emblem Theme, Anna trophy in the 3DS version
Only Jugdral and Sacred Stones have no character. Marth, Roy and Ike are the main protagonists of the first games of their respective sagas. Awakening gets two of the three protagonists (Robin and Lucina), with Chrom being part of Robin's Final Smash.
Of note is how FE characters have alt colors based on other characters from the series. Marth has a Roy recolor, Ike has recolors based on Greil, Black Knight and Chrom, Lucina has recolors based on all your waifus Nowi, Tiki, Tharja, Cordelia, Cherche, Lissa and Sumia, Roy has recolors based on Eliwood, Marcus, Zephiel and Cecilia.
Have I mentioned that I kinds miss the stickers from Brawl? Collecting them all was a chore, sure, but they could at least have put an art section since I liked the idea of having art works from the games represented in Smash.