My two cents on fixing Radiant Dawn:
Part 1 glossed over quite a few key battles in Ch8's world map sequence; the siege of Talrega, most notably. One of FE10's major problems was that the split party approach yielded unequal results - part 1's units start at a lower base than 2/3rds of the rest of the cast, so you grind them up to barely reach t2 and they still end up largely behind the difficulty curve. This is especially notable with Fiona, who joins at the start of Ch7 (instead of mid-battle in 6-2, for whatever reason), and the next four maps are: indoors, swamp, indoors, riverbank - her stats don't really make up for the hassle, either.
The Dawn Brigade could probably use another Chapter or two in part 1, and an additional chapter in part 2 that would cut to the reconstruction effort, perhaps involving Bastian's diplomatic mission in some way. We also know that Izuka's "disappearance" after part 1 was really Volke kidnapping him for interrogation. Perhaps that could be pushed back until part 3; one of the major weaknesses was the whole "Blood Pact" subplot, which "turning everyone into stone" eventually subverts anyways. Keeping Izuka in the picture, especially with how much personal faith Pelleas has in him, would give Daein a reason to enter the Begnion-Gallia conflict - stirring up the country's historic racism toward the Laguz, while plotting to build a feral army. When Volke catches on, Izuka gets hauled off to Bastion's manse. The origin of Daein's Blood Pact is absolutely cartoonish - the treaty signed at the end of part 1 was secretly infused with a magical plague and a servitude clause? Did anyone (other than Izuka) bother to read that part? It's Saturday morning trickery, at best.
Instead, Izuka goads Pelleas into the alliance, and once the fighting's already started the Laguz Alliance views Daein as an enemy. Volke kidnaps Izuka shortly afterwards, which sends Pelleas into a bit of a panic. Lekain warns that the Laguz will raze the entire nation if Pelleas fails to cooperate, instead of "hahahah you have to do whatever I say because you signed magic paper, dumbass." Pelleas is green enough around the gills to get bullied into a stupid alliance. And let's forget about that corny "I'll kill myself to get out of this bad contract", Mr. King of Daein. Also, Ashnard's pact never needed to exist in the first place.
The Kilvas pact is similarly hamstrung - how was Naesala serving Begnion in FE9? He doesn't even really do anything particularly villainous in FE10 other than leading Phoenicis troops into a trap, because Naesala realizes that he can subvert his Blood Pact by serving Sanaki (which technically counts as "serving Begnion"?). You'd think Lekain would be smart enough not to include such a silly loophole regarding a person that he clearly planned to usurp. It totally cheapens Naesala's anti-hero role - he could've just stayed neutral and sarcastically cited Goldoa's example. Perhaps Kilvas is grappling with a real life plague, instead of that Blood Pact nonsense?
Lekain proclaims that the true Apostle, an older sister to Sanaki, was assassinated alongside Apostle Misaha. At the same time, Chancellor Sephiran is denounced for perpetrating the cover-up of the true Apostle's identity and captured. With the Tower of Guidance shining radiantly, Lekain announces that Begnion's enemies shall be judged by the Goddess Ashera herself. Forming the "Disciples of Order," a military force under the Senate's direct control, Lekain raises an army of his own to usurp Sanaki and take control of the Holy Empire.
Tormod, Muarim, and Vika should probably join the Laguz Alliance instead of randomly popping up at Oliver's house. Or they could at least show up in the Grann Desert, because that's where they live?
Part 3 could end much in the same way, sans the "judgment of Ashera" part. Micaiah sings the galdrar, calming the Medallion's chaotic energies. Perhaps the Goddesses could still descend, but without using Ashera as the final boss because the original direction of part 4 is pretty universally disliked. Upon her awakening, Ashera keeps her promise to consult Yune, the Beorc, and Laguz, before passing judgment on humanity. Yune knows that she must convene with Ashera at the Tower of Guidance, which is why Micaiah forms a truce with the Apostle's Army.
The fact that Sanaki was unable to do this confirms that she is a "false Apostle." Word begins to spread about the Maiden of Dawn's miracle; Lekain takes advantage of the fact, denouncing Micaiah's upbringing and declaring her to be a mere gypsy and follower of the Dark God. Lekain is the first to pray at the Tower of Guidance; he asks for Ashera to eradicate the Holy Empire's enemies as reward for Begnion's faith - Lekain's greed and arrogance leaves Ashera somewhat convinced that the Beorc and Laguz should be eradicated. Having been judged unworthy by Ashera, Lekain and his companions are incapable of ascending the Tower of Guidance. Sephiran, aka Lehran, having been freed from captivity by Zelgius, enters the Tower undetected to advise the goddess - Sephiran tells Ashera of the Serenes Massacre, along with the many other atrocities' he's seen, and further convinces her to start the world anew.
Meanwhile practically the whole of Begnion has rallied against the "Dark God's Army." In order to move their troops into Begnion undetected, the alliance of Daein, Crimea, the Laguz Alliance, and Sanaki's loyalists split into three parties. Ashera bemuses the fact that Yune, her other half, is portrayed as a vile creature and used as a rallying cry to raise an army. Sanaki's allegiance with Micaiah further reinforces the "false Apostle" narrative that Lekain's been selling to the people. Dheginsea and the Goldoans have some sacred shrine that serves as a warp point into the Tower. Ashera telepathically spoke to Dheginsea, informing him of the impending judgment, choosing to spare the Goldoans and eradicate the rest of life on Tellius.
Blah blah blah, they all fight their way to the Tower of Guidance and regroup. Go inside. It's revealed that Lekain was responsible for the Serenes Massarce, so kill Lekain. Include a way to spare poor ol' Hetzel so that he can join instead of randomly resurrecting Oliver. Under the direction of Sephiran, who has lost hope in the world, Ashera's personal army fights off Yune's allies. Your units can ascend the Tower with Yune's blessing. Kill Zelgius. Kill Dheginsea. Defeat Sephiran (give him goddess blessing superpowers or something to make him the final boss), after which he's convinced by Yune's mercy that the world ought to be preserved as is despite the obvious flaws, and that time shall bridge the divides and ease the tensions that have plagued Tellius. Sephiran and Yune pray to Ashera, and the Tower's glow fades away. The two goddesses reunite, and the world is saved. Everyone lives happily ever after (except Oliver because he died). The End.
I liked what Radiant Dawn was going for, because the omnipotent threat throughout the entirety of the story is centered around Lehran's Medallion; there's too much time spent building that up throughout the course of the two games. Ike's chapters at the end of part 3 felt like a real "race against the apocalypse" scenario. What came afterward was poorly executed, most certainly. Ashera's antagonism right out of the gate was super contrived, and there were some unnecessary, um, "flourishes" masquerading as key plot points (blood pacts cough cough). These are things that can be resolved without throwing the script out the window, though.
And for fuck's sake Ranulf, don't spoil the Black Knight's identity.