Filters and fluids! Change, monitor and maintain. Aftermarket radiator cap. Aftermarket exhaust and air intake. Banjo bolts. That is the list I was given and am working on as we type. Fear not, the 6.0 is an awesome truck, like any that you don't kick the **** out of!
Filters? Motorcraft only. Fluids? High quality. Most run Rotella T6 oil.
Replacement parts? For this engine there is nothing better than the factory.
Air intake? Uhmmm, no. Don't waste your cash on those. Your stock air intake is a true cold air intake (it gets its air from in front of the truck). Those aftermarket "cold air" intakes take air from under the hood, which is anything but cold.
6.4 Banjo bolts? Not worth the time/effort/cost to change. A whole lot of "blah blah blah" about nothing. Leave them be.
Updated fuel pressure regulator (aka: Blue spring).. Look it up and do it.
Gauges, gauges..you NEED gauges. Engine coolant temp, Engine oil temp, trans temp, boost, fuel pressure. All important. EOT, ECT, trans temp and boost can all be electronically monitored through the OBD port. You can get a dongle and use the torque app if you'd like.
You will need to add a fuel pressure gauge. Yes, it is that important.
In my signature, click on the "basic info for the 6.0 owner" and give that a read.