Nice spec! Don't see many double cabs, but I think the proportions are better, and the 6.5ft box is definitely a big deal over the 5.5ft....I find a 5.5ft is small, even with the tailgate down, for nearly everything I try to haul...
As for accessories - I did a build on a Tundra for work a couple years ago; mostly aesthetic stuff for a magazine shoot, but I do remember that there was a TRD rear sway bar, which I ordered directly from a Toyota dealer, that came highly recommended from the Tundra forum and really tightened up the backend. If memory serves, ours (a 1794 edition) had halogen headlamps that were terrible so I go some aftermarket projector assemblies and ran LED bulbs.
I am a little foggy on exactly how this worked, but I recall ours (a CrewMax) had like a carpeted foam hump under the rear seat, useless space, so I order a kit from somewhere that bascially had me cutting out a section of that carpet/foam and inserting a couple plastic storage bins. I'll have to see if I can dig up a photo because the end result was pretty nice, and provided a lot of extra storage. Just not sure if the DC is the same back there or not.
Other than that, I'd tint the front windows to match the rears and get a nice set of in-channel vent visors for it - but that's more of a personal preference than a real recommendation.