All depends on the kind of car you want it to be. Do you want it to be louder and flashier, or do you want it to handle really well? If you're going for handling or performance, a good place to start is always suspension and brakes, to handle the extra power you'll be eventually throwing at it.
I wanted a strong car for the mountain roads, so I went with subframe bracing and suspension first, since the Performance Pack gave me plenty of brake for the power. Steeda has a variety of good parts for this currently. K-Member front brace, strut tower brace, rear subframe connectors, springs, swaybars.
Some kind of a tuner is a great idea too. For the EB, I went with Cobb, but there are a variety of tuners out there for all the engine variations that make a noticeable difference and allow you to continue tuning it in the future as your mods change.
A cold air kit or higher flow panel filter and an axle-back or cat-back exhaust are logical steps as well.
2015 Mustang Ecoboost Premium w/ Performance Pack, 2013 Fiesta SE hatchback