My theories on what they could do:
-Stats are separated like in Fire Emblem. HP is the same, Attack/Magic determine damage done (attack/magic+Weapon might), Defense determines how much damage will be taken with physical weapons, Resistance with magical weapons, Luck determines items drops. Though I'm not sure what to do with Speed or Skill.
-Characters will be alternate costumes for different classes with some minor differences in movesets and weapons. For example Soren uses Wind magic which deals bonus damage to fliers, Sanaki does the same with fire magic, Orochi uses scrolls, etc. Weapons are also unique for each character and different charcaters can uses multiple weapons, for example Ike uses a Steel sword>Ettard>Ragnell OR Steel Axe>Silver Poleaxe>Urvan. Chrom with Steel sword>Rapier>Falchion OR Steel Lance>Spear>Gradivus.
Different classes also have different dash speeds, guard meters, combos, and stats.
-There will be an Affinity system like Hyrule Warriors and Radiant Dawn. Different maps give bonuses to certain characters if the map and character have the same Affinity. This also effect certain attacks like what we saw Chrom do.
-Different characters have special attacks that fill up a Bar like in Hyrule warriors. These special attacks could work similar to Heroes where it has an AOE, deals more damage based off of a certain stat, etc.
-Bonus damage deals more damage and can break through guards faster than a standard weapons if used on the appropriate class. Wind magic with fliers, hammers with armor knights and beast killers with horses and beast units.
-Mounted characters will have their own movesets like Epona from Hyrule Warriors. They can not use dodging maneuvers however their default speed is comparable to a dash and have a form of super armor during certain attacks unless hit by a weapon that deals effective damage against horses.
To add onto this wyverns and pegasi will be playable as well. They also fly a few inches over the ground.
-Armor knights have the slowest running speed in the game but their guard can only be destroyed with effective weapons.
-Shapeshifting units like Manaketes, Laguz and Taguel could work in two different ways: They have a special attacks bar like in Hyrule warriors(The yellow bar) that builds up when they hit or are hit. They attack with standard punches and kicks until the bar maxes out where they can transform into their dragon/beast forms which receive a large attack, speed and defense boost until the bar(s) runs out. Or they transform by default when the battles starts.
-The Weapon triangle will be in the game and it will work like it always has: Swords>Lances>Axes>Swords. Wind>Fire>Thunder>Wind. Anima>Dark>Light>Anima.
When fighting a major opponent like a General (Not the class) or named character with a weapon advantage you deal more damage and a special attacks bar increases faster. However if you have a disadvantage it does the opposite, you deal less damage and the bar will fill slower. The enemy can also do the same to you. Though this will not happen against smaller grunts.