Don't count on Ike or Roy for Fire Emblem Warriors 1 DLC. According to a Nintendo Dream article, they won't make it in unless there's a FEW2— Source Gaming (@AllSourceGaming) September 30, 2017
According to an interview (translated by Source Gaming) in Nintendo Dream, it looks like Ike and Roy don't have much of a chance to appear in Fire Emblem Warriors via DLC. Looks like the only hopes for them getting in on the action would be a sequel, which will certainly be a little disheartening to fans. Let's hope this entry sells well enough to warrant another installment!