Just a couple of things that pop to mind, mostly for the near stock versions of the XJ. In a roll over without a cage, the A pillar folds pretty darned easy. You can look at the A pillar and the door from the side and see where the engineers were headed. As long as the door is closed, most of the the strength for the A pillar is provided by the door. The problem is, just the weight of the vehicle on the roof, pops the door top out and then the A pillar folds.
It wouldn't take a genius engineer to develop some sort of catch for the door top, so if it does (start to) pop out of the door frame it has an additional mechanical device to help keep it in to door frame. And/or to help the roof capture the door top as it folds.
Idea number two. A replacement splash shield (tin or aluminum) for under the radiator with a better seal and more surface area, to help limit mud, water or whatever getting into the fan and alternator. Maybe something with Klico (sp) spring loaded screw slot fasteners for quick removal.
Here is one for the Jeep Speed types, a coarse matting material, that really doesn't mess with air flow, but that can be installed behind the grill to protect the radiator form stones and brush.
Some kind of hard foam bracing for under the radiator. Most every XJ radiator I've ever seen that is on a well used vehicle, has a smile bowed into it. Two bottom mounts aren't enough.
A roof rack, with roof light guards built in. Maybe even a flip down (metal) cover for the lights, to make the Police happy (for the states that require the lights to be covered on the highway). One cover for all the lights, flip the handle and the cover goes down and/or up. Maybe even place the handle so it can be operated from the drivers seat with the window down.