Acclaimed Flop: Both games got high critical praise upon release (though slightly more mixed in Radiant Dawn's case) and are still loved by fans now. Commercially, both games bombed and their poor sales were influential in Fire Emblem's near-demise.
Bad Export for You: For some people, the dumbed-down script of the English version of Radiant Dawn and the removal of Maniac Mode in Path of Radiance makes these games fall under this trope.
Franchise Killer: For the home-console Fire Emblem games, at least. Since Awakening, it seems that portable systems are the way to go now for Fire Emblem.
Keep Circulating the Tapes: Both games, but especially Path of Radiance, are infamously expensive to get hold of these days. It doesn't help that both games sold terribly when they were released, but were widely praised by fans. Now that Super Smash Bros and Awakening have made Fire Emblem more popular, interest in the Tellius games has skyrocketed but the games themselves are extremely hard to find. Many are clamouring for a Virtual Console release.
Several S Rank weapons, like Urvan and every ultimate magic tome except Thunder and Light, as well as the Beacon Bow, a light magic-based bow that would've been able to attack from melee range, are among unused weapons in Path of Radiance.
Path of Radiance has unused data for playable incarnation of Greil, complete with set of new base stats(much higher than what he has in Prologue), growth rates, and even his own animations for Aether. A common theory is that he was supposed to be a Trial Map unit.
Radiant Dawn contains data for specific support pairs, but not actual conversations. It's speculated that they were still going to go with the "everyone supports" system but give certain pairs unique conversations for the A rank. Sadly this ended up being dropped. Path of Radiance also has a ton of unused support pairs; most notably, Elincia and Volke were meant to have a lot more support options (Volke only has one in the final game) and Zihark has unused dialogue in the epilogue that was supposed to show if he had an A Support with Ike.
Some leftover bits of data in Radiant Dawn, plus the fact that male Swordmasters are the only second tier class capable of reaching the SS rank in their primary weapon skill, indicates that Stefan was at some point meant to join the Dawn Brigade in the first act.
Unused dialogue for Part IV Endgame of Radiant Dawn indicates it was at some point possible to spare the life of Hetzel. His fate wouldn't have been much more pleasant, though, as Sanaki would immediately order him arrested and charged with high treason after the battle.
Information (that isn't even officially translated!) gives the backstory of the Dawn Brigade.
A Q&A from the Japanese Path of Radiance website gives the average lifespan of each type of laguz. 5-6 times the beorc average for the beasts, ten times the beorc average for the hawks and ravens, a thousand years for the herons, and two thousand years for the dragons.