I could list my choices, but I already had a whole video on a similar topic. Instead of just stating exactly what I already stated in my video, I will put 5 characters I personally would choose if I had control of the roster and put a TL:DR note for each:
: Was one of the last characters finished, would be very low priority again.
/
: I wouldn't want to add Corrin back when there's several FE reps to choose from that are far more deserving. Fates did poorly, and this character was so poorly received.
: Roy is not my boy, in fact, I greatly dislike Roy, so please go away and don't come back another day.
/
: Good joke character for Smash 4, sure, but would I bring 'em back? Nah.
: I don't care if this is against the rules, but I don't want Miis in my Smash. Custom moves failed, so in essence, Miis did as well.
This is why I'm not in charge though, as there's a reasoning behind the cuts in the first place. I narrowed past cuts down to 4 points in particular:
1. They're a clone/semi-clone of someone else. Ex. All Melee cuts (bar Mewtwo).
2. Their inclusion doesn't work with new mechanics for the new game. Ex. Pokemon Trainer and Ice Climbers.
3. They have license issues that prevent their return. Ex. Snake, because Konami/Kojima.
4. They don't have time for the character, or they just aren't high up in their priority list. Ex. Mewtwo lost out to Sonic.
Final thing to consider:
The 64 and Melee vets are safe, as well as most of the Brawl vets. If they made the cut to Smash 4 in the first place, I doubt they'd be cut, especially with Sakurai in charge.