Pre-July 14th, my answer for this every time would be, "What are all of the playable Fire Emblem reps in Smash Bros for 3DS?" That's seriously all that I really cared about.
However, now that all of my wildest dream have come true, I guess my question would be, "What is Dark Pit's role in Smash Bros for 3DS?" I'm genuinely curious. Being an alt costume with his own voice would be fitting and fine, but after Lucina, in Chrom's words, "Anything can change!" It also seems odd that he was teased at E3 but we haven't seen anything about him since then.