Apologies, I've not had time to progress this at all for a while. But what I have been doing is driving my 435d xDrive for a few weeks. It has very sticky way wider tyres than the i3 and the steering is light as a feather, always returning to centre. Can be steered with light fingertip pressure. At the moment I'm not sure how widespread this problem is. There seem to be quite a few people on here with the problem but how many are out there without any problem or not realising it is a problem. I'll be honest it took me a few weeks of driving to realise that it's not just a characteristic of the car and that there is a fault. This was confirmed by a local steering rack reconditioner.
The steering system is no different on an i3 to the vast majority of cars out there that are under 15 or so years old. It's electrically assisted rack and pinion with a torque sensor. At the moment I'm not at all sure whether it's a physical sticking problem or whether the torque sensor/electrical system is fighting against the natural tendency to self centre.
I'm getting to the stage where I might just take the rack to the reconditioner who assures me it will be 'fixed' and see how it comes out. It'll cost me around £300 + VAT though and I'll be none the wiser what was actually wrong unless they tell me.
Worst case scenario they recondition the rack and it comes back just the same, then they tell me it's normal!
There have been people who have taken their car to BMW under warranty and been told the rack is corroded and a rack replacement is the only cure.
If I was to make another observation, I feel it may have got worse since the weather got colder (down to around 0C). Not by a huge degree but it's a feeling I get.