I have like, several propositions for this, but I am not sure if you can handle them. Anyway, get ready for a ride, I guess!
Jerrot/Zealot is now Marf (Santagelo) (That Eliwood recolor with FE7 Wil's palette) from Ostian Princess, has Arran's bases from Shadow Dragon, but Seth's growths from FE8, with +15% on each growth but Resistance. Carries a Silver Sword, a Steel Lance and a Vulnerary. R-Text: "A traveler of sorts. Friends with some abomination named Zanatta, and some Lilian person.". Death Quote:<Marf> "B-But I am Marf Santagelo...! I'm too handsome to die-" <Dieck> "Cut it out, man. You're already dead."
Fir is now Marisa but with Mareeta's Growths from FE5, and Marisa's Ephraim Route bases. Wields the Wo Dao and a Elixir. R-Text and Death Quote are the same as FE8, but no longer supports Karel. (A rather dicklish nod to Ewan recruiting her even though they don't support each other).
Miledy is now Ephraim from FE8, same class and bases as Miledy, except he only starts with Gant's Lance, a Javelin and a Elixir. Death Quote is from FE8. R-Text for Ephraim: "The current prince of the Kingdom of Renais. Actually working as Guinivere's escort."
Zeiss is now Eirika from FE8 as well, starts as a Level 1 Wyvern Lord, but starts with Glen's bases from FE8, and has Est's Growths from Shadow Dragon, but the Str growth is 35%. Her inventory consists of an Al's Sword, a Light Brand and a Goddess Icon. Death Quote is the same from FE8. R-Text for Eirika: "The current Princess of Renais. Assists Ephraim in escorting Guinivere."
Yuno is now Sigune, same bases (except Luck, which is 9), but starts as a level 10 Pegasus Knight, has Palla's growths from Shadow Dragon, carries a Silver Lance, Iron Sword and a Elysian Whip. R-Text and Death Quotes are unchanged from Yuno's.
Dayan is now Joshua from FE8, but with Carlyle's Hard mode bases, FE7's Karel's growths, class is now Swordmaster and carries a Wo Dao and a Runesword. Death Quote is the same as FE8, and so is his R-Text.
Dory/Dray/Drei is now the infamous Money Man. Same bases, but with 7 Luck. Inventory unchanged. R-Text: "A helpful, but greedy old man who lives in Pherae- Wait a second...". Battle quote: "Hey you, guess what? Money is important, damn it! .. Not as important as lustful affairs, though... Er... anyway, to the point! Fight me and you shall see our master!". Death Quote: "Money!! Money is important, damnit...! Without it, your weapons will rust... and you can't fight without your weapons...! Argh...!"
Oro is now Beran from FE8 with his Hard/Difficult Mode Bases, change his class to Nomad Trooper (perhaps even higher for this game's Hard Mode, about 1-2 points more for these bases, +5 speed), same inventory (except the Lancereaver should be one Al's Sword/Killing Edge), and same R-Text, Battle Quote and Death Quote as FE8.
Change Roberts (A.K.A. "The only Hard Mode-exclusive boss in the entire game IIRC") to appear in Normal difficulty as well (unless that's hardcoded or something...), and/or change his class to Valkyrie and give him Bolting and Fimbulvetr, with Kempf's bases from Thracia 776 as well.
Sigune is now Valter, with his Hard mode bases, class is unchanged/Wyvern Lord and wields a Killer Lance, a Spear, and a Delphi Shield. R-Text and Death Quote are the same. Battle quotes are from FE8, and his exclusive battle quotes now happen with Eirika and Ephraim only, having none for Jerrot or Marf.
Gel now has nearly-maxed out luck (around 25-28 should be fine), a Wo Dao and maxed out Strength. He smiles in his portrait now. R-Text: "Perhaps the most hated Swordmaster boss in any GBA FE other than Carlyle. Fight with caution."
Aine has Morzas' bases from Shadow Dragon.
Gale is now Lyon, except he spawns warping in except of "diving down" (although the Wyvern Lords around him still spawn in the normal way). Bases from FE8's Hard mode, carries Apocalypse, and a Elixir. Battle for Whose Sake plays for him. Eirika can still talk to him, though. R-Text: "The former prince of Grado. Warped into Elibe after screwing up in a summoning ritual.". Death Quote is unchanged.
All of Pereth's quotes, battle quote and R-Text are changed to "You can't win against Pereth!". His stats are maxed out, too. He also has the portrait of the buck-teethed bandit. Death quote: "You... can't win... against... rgh.."
Yahn has Medeus' Bases from Shadow Dragon's Hard 5 difficulty, and is accompanied by two Sages with Forblaze, yet they have Veld's bases from FE5.
Gant's Lance and Al's Sword have the rapier's effectiveness, have the Überspear's Stats (uses are 25 instead, however) and it's stat bonuses (only no HP bonus). It's sold in every secret shop in the whole game for 5000G each.
Maltet is now a 1-2 Range Lance.