Meant in hypothetically, though if they REALLY wanted to add him, they could always swap him out for Frederick or Morgan while Chrom's fighting.
Simple, really. A more "athletic, rugged" style. Lots of kicks in the air to complement swordplay, very high jumps (seriously he jumps in like half of his Awakening animations), slower-than-Marth-but-still-fast. Chrom's brash in his training. Add some charges with his sword, as if it were a lance almost. Speaking of lances, Chrom gets them upon Great Lord promotion. Add a Javelin-toss Side Special. If he must have a Counter, make it an anti-air, using the defensive pose used in the cutscene outside the Feroxi gate with the spears. My Up Special plan was always similar to Sword Scabbard with more control going down, but eh, that's good enough as it is. Neutral Special? Holding up the Falchion and summoning Naga's Power to wield the Exalted Falchion. This adds more range to his attacks by giving them a holy burst effect. Hell, even us the Shield of Seals to use Holy Fire in a smash attack somewhere. Shepherd Assault for the Final Smash. OR ALTERNATIVELY, sharing Robin's, with ROBIN coming to assist, or Frederick if both are in battle. Have him focus on juggling and rushdown.
Alternatively, a weapon switch mechanic between the Exalted Falchion and a Spear, adding more of the strategy aspect of Fire Emblem and the Weapon Triangle.
He's FAR from without options.