Just my own wishful thinking, but maybe they could have the final chapters of previous games, where Chrom and his army takes the place of the army that assisted the main lord of the previous game, and they relive the fight, with unique models for the bosses and old main lord.
For example, the final chapter of Rekka no Ken, Chrom and his army are under Eliwood's command fighting against Nergal and his Morphs. Eliwood has a unique class heavily resembling his Knight Lord class, and Nergal is the same, resembling a Dark Druid. Hector has a Great Lord class and Lyn has a Blade Lord class.
Other DLC chapters would be the final bosses of other games, with their main lords being the commander of Chrom's army.
Rewards could be an item similar to Dread Scroll and Wedding Bouquet, in that it allows specific characters to promote to a new class. Different items would be released for each chapter, granting the DLC/Spotpass Lords of that chapter the chance to promote to their old promoted Lord design, with matching attack animations and such, but all of these items would also work for Chrom and Lucina, because they're both Lords too. Also, the villain of the chapter (In this case, Nergal) can also use the item to promote to their old class as well.
I just think something like this would be nice because it allows players to understand the main trial older generations in the Fire Emblem world had to face.
I also think the reward would be nice because my nostalgia! It was something I felt awkward about when the DLC was first released. I mean, I understand that they wanted to help players feel more comfortable with the old characters/new class design by having special artwork done for some of them, but it still just isn't the same. Some characters didn't even get this treatment... I remember Hector for being a badass swinging axes around single-handedly, not for the extreme amount of armour he had, struggling to run at the enemy. I remember Ike for being a mercenary being made to command the army of Begnion, not for being Mr. Roboto. As for class abilities, they could easily throw existing abilities to those classes. Aether, Aptitude... Knight Lord could even get a redesigned Wyrmsbane that works for that class too, instead of just Manaketes, because of how Eliwood had to fight the Fire Dragon at the end. On top of this, why not give Ike's Hero Lord class the unique Aether animation he once had?
Finally, I understand certain animations, mainly Lyn's Blade Lord animations, might not work so well in the Awakening world, because the disappearing ninjutsu stuff is completely out of place. But I still think small adjustments could be made to resemble what her old animation was like. If we made Lyn's critical animation look like Astra from Path of Radiance for example... Lots of blinding speed, but without the disappearing blurs and stuff.