I'll add a few avenues to reinforce newhue's considerations: security is a main concern of people. There are mechanical commercial locks available that can be retro-fitted but invariably are not defender-specific and require a lot of bodging to get to fit. I've had the need for particular shaped channels and plates that would enable certain lock types to fit, either as support for the lock mechanism in the door, or as support for the striking plate in the frame (or of course vice-versa). My work-arounds have done the job, but often been ugly, obtrusive or both. So there's a need for mounting plates, receiving plates,and all manner of other things to hold doors closed.
I'd like something that allowed the steering wheel to be easily removed and then some kind of brace that fastened to the two front doors preventing them from opening and went over the steering column boss as well so the doors cant be opened and the wheels cant be turned. If it was elegantly made so it looked like part of a top-dashboard assembly and could sit there out of the way until needed, that would be brilliant. Lots of flat space to laser-cut logos and adverts on it too. Park up, pop off the wheel, shove the bar across fitting into each door on each side and with a lock on the steering column, and exit out the back door. Or simpler - again with a removable steering wheel something that goes from the steering column to the gearlever preventing steering or gear movement, and a lock at the steering boss. (Of course only useful for non-airbag older vehicles.)
That would be fairly inconvenient for a thief. Of course you need a good lock on the back door and if you take a look at it you'll see it is actually easier to fit stuff in there, but again it will need ingenuity. I'm not mentioning specifically what I've got for obvious reasons.
Looking ahead defenders are probably going to become scarcer and therefore more of a target for thieves - certainly in certain parts of the UK defenders are hot targets for thieves because they are (relatively) easy to get into, easy to strip to parts, no-one notices them being driven down the road as they are fairly common, and easy to sell on the s/h market. That's an opportunity for you perhaps.
External stuff - I'd like some small guards that can be fitted on the side repeater lights on the front wings, I keep smashing them, so a little metal receiver for them to stop my boot or elbow from demolishing the wee orange plastic light cover would be good. Of course you can take them off and plate the hole over but they are very useful lights so I'd like to be able to use them!
A replacement front door handle made of metal with a slightly deeper handle and cross-hatched grippy top and strengthening plate behind so you can stand on it - that'd be good. NOBODY makes them.
Think that's a daft idea? Well over here one agency I work for, and that runs a fleet of defenders, spends more on door handles each year than replacement shocks and springs because the deerstalkers and other staff use the door handle to climb up to the roofrack, and breaks them. You cant leave it broken and 'just get by' so they have to be replaced immediately. Door stripped, handle replaced, door rebuilt. Time & parts = cost.
Combine a metal doorhandle with a new lock plate/padlock/locking pin system that goes over across the edge of the door and adds an extra, tough lock, and you have a multi-use device. Incorporate a bottle-opener as well and you can sit back and smile! Same on back door area - something that protects and makes a step out of the numberplate light with protection for the light (and another bottle opener) would be good.
And there's lots more I could suggest, but that's enough vapourware from me. I'm off to bend an old piece of scrap with a hammer to make 'something' security-related for my back door!