I don't see why we could have most/all of the Greil Mercenaries. Who wouldn't want Titania in this game? Best Jeigan ever.
WHY CAN'T WE JUST HAVE ALL OF THE CHARAcTERS
Because there's like 500+ of them and many are archetypes of each other with a couple of blatant pallet swaps thrown in.
There's 78 different characters in New Mystery alone (though that includes literally everybody in Shadow Dragon short of Lorenz (who blows himself up/dies on his sword, depending on the version of MotE), Gotoh (who got demoted to NPC/exposition dump), Malledus (who's your captain exposition for SD and gets replaced by Jagen who got too old for this shit and needed something to do), a bunch of one-off bosses, Medeus (who's the big bad), and the 'replacement' characters (like Wymp, Owend, Laim and so on) which brings Archanea's playable population up to 80).
Radiant Dawn by comparison has 72 characters (Path of Radiance had Largo as well as anybody from Trial Maps not named Oliver which gives Tellius a grand total of 77 units).
Fates has a grand total of 67 units over three games (69 if you count the different genders of Corrin & Kana; and that's not counting anybody you might capture or Bond unit shenanigans).
Binding Blade is next with 63 units with an additional 40 to Blazing Sword (which gives Elibe the third largest overall playable population next to....).
The combination of Genealogy of the Holy War's 56 (including subs because they count dammit) and Thracia 776's additional 51 (for a grand total of 107).
Awakening has 49 units not counting the DLC characters (51 with Robin's and Morgan's separate genders of course).
Sacred Stones has 43 total (though most come from Creature Campaign; only 32 are normally recruited).
And Gaiden is at the bottom of 28 unique units (32 are playable but four of them are refugees of Archanea which brings the continental total to 108, edging out Jugral for most PCs).
And there's TMS's losers like Tsubasa or Tiki is my Waifu, the OCs from that Hasha no Tsurugi manga (though they perhaps don't count since they aren't playable yet), and Cipher's 4 OCs, on top of the villains that weren't included in the above (although there aren't many thanks to all those Trial Maps).
Meanwhile, Zelda fans are still waiting for the Groose to get loose, let alone anybody showing up from the Oracles or Minish Cap. And Dragon Quest Heroes still haven't even touched DQII, nevermind the rest of VII's & VIII's cast or IX's premade bonus characters (i.e. the innkeeper, angel trainer and other bar staff). Hell, even Warriors Orochi 3 Ultimate only has 149 characters. Most came from across 11 games plus a bunch of crossovers like Ninja Gaiden's Ryu Hayabusa, Bladestorm's Joan of Arc and Soul Calibur's Sophitia.
In short, they're not getting to everybody with even three games + whatever they're calling their not-expansions nowadays, let alone one.