Any well-equipped four-by needs an illegal number of extra lights. The front bullbar should be able to take four, but don't stop there when a rooftop light bar can take another four or more. You'll want a giant backup light and a couple of big work lamps hanging off the back of the roof rack you're eyeing. It doesn't matter that you'll rarely go of-roading at night; they need to be there just in case.
Rip out those stupid standard seats and replace them with full harness competition units; you'll need them anyway for the off-road racing you might get around to one day. Same thing with a roll cage; just get it now. Manufacturers never put in enough instruments so you'll want to add some, leaving no aspect of the rig unmonitored.
Real four-wheelers need to carry lots of stuff, sometimes to actually use. Ropes, shackles, several pulley blocks, a high-lift jack, vehicle parts, tools ... and some storage setup to keep them safe, clean and ready. You could make something, but what would be the point when the aftermarket is ready to feed your need?
There should be an electric refrigerator large enough for a week's camping, in case you ever do, and an enormous power inverter should there ever be a need to operate high-draw electrical gear on the trail.
Communications gear is a good way to spend money. A modern UHF radio and an old fashioned AM CB just in case you meet an elderly Land Rover are good starters; seek out a mobile communications expert and a whole new world of potential purchases unfolds. A rescue beacon's a must and while we're on safety gear, there's the first-aid kit, several fire extinguishers, fluoro vests and why not a hard hat in case you have to chop down a tree with the chainsaw you'll be adding?
Think of the engine as an unfinished canvas. If the engine isn't turbocharged, it should be. Or supercharged, or both. If it has a turbo but no intercooler, fit one. Consider an engine swap; a Lexus V8 is always good. Further along the powertrain, you'll need front and rear diff locks and a mighty wired-in air compressor to run them, inflate the giant tyres and maybe even work a rattle gun.
By this time you'll be in so deep that you might as well buy a snorkel for the engine so it can breathe in water. You might be up the creek without a paddle when the wife sees the bills (see helpful breakout) but the engine will be okay.
How to get it past the Significant Other
* These extras aren't toys, they're investments that will enhance its value.
* It'll be way safer for you and the kids.
* It's our ticket to safety in an earthquake/tsunami/plague/the driveway collapses.
* We can get into the outdoors more often.
* We can use the winch when we cut that tree down.
* And: Always leave off a zero when discussing prices.