Put it this way.
There are four logical reasons to add, or withhold, a character.
1. Relevancy/Fanbase/Reaction.
Basically, if a character is relevant enough (ie Joker) or has a big enough fanbase (ie Banjo) then it has some merit as a roster pick. Then again, Piranha Plant and Wii Fit were added as surprise additions, to get interesting reactions.
2. Gameplay.
If a character is fun to play, and has an interesting moveset, then they make sense. Look at Bayo - she plays pretty much as she does in her original game. It's also why characters like Wii Fit are interesting, because it works based on the source material in an inventive and original way.
3. Legal Stuff.
If licencing can't be solved, the character's not likely to get in. That's why Bond wasn't in Melee. However, it can (in cases) work the other way too, I suppose.
4. Development.
Clone characters are easy to add - hence Falco, Ganondorf and Pichu being in at all to an extent. If a character is easier to develop, then they make sense as a roster pick - so having characters like Lucina and Chrom makes sense.
There's no secret Fire Emblem agenda, nor is Sakurai actively trying to piss you off - he is sinking his heart and soul into pleasing millions of fans across the globe. So what if you were salty at Dragon Quest representation, or Fatal Fury representation - there were people who had dreamed of characters like the Luminary or Terry to be in Smash. That's why K. Rool is in the game, and Isabelle, and Ridley, and why everyone returned in Ultimate, and why Sakurai even makes any effort at all - Sakurai wants to please people.
Anybody else remember the times when we were all excited that Ridley was actually getting into Smash, regardless of whether we were Metroid fans or not? Of people not being salty because a character got in? That's what this speculation should be - fun, not arguing about the supposed oversaturation of Fire Emblem content for the 9000th time.
Sakurai isn't personally trying to offend you, he's trying to please you. He's not trying to start an argument - in essence, Smash Bros is a way to end arguments! Just... don't take the existence of franchise representation in a crossover game personally, okay?