Save for remote cam modules, no single form factor works well for everyone. LH or RH drive, factory sensor modules, mirror placement, frits, windshield tint strips, sun visor placement and more will always keep this the same. And even remote cam modules may have cabling in the wrong place for your car, plus those have the issue of where to put the head unit. So there is no "best" or single solution, but some form factors are more versatile than others, and this is even before considering 'stealth' from the outside view.
One thing which I think would help everyone is to have cables using the same port format on both ends with a 90 degree connector on one end and a straight connector on the other end Then at least one end could be optimized without the need for any adapters which themselves can also cause problems with ftiment.
I don't see LCD screen placement as much of an issue for most users, whose cams will be used more like 'set and forget' devices than ones needing constant attention. But there are enough users wanting to turn parking modes on and off to make seeing what you are doing with that a consideration.
This can be handled by button placement and LED indicator flash patterns or colors which I think is the best approach But there's little point in a screen if you can't see it and in some cars the mirror can't be moved enough to have a really good view of a screen anyway. I think button placement is far more important than anything relating to screens.
If there's one drawback to the A139 form factor it is one shared by almost every other cam, and that is not having the ability to aim the lens horizontally so that you can still get a straight-ahead view even with the cam mounted to one side of a very curved windshield. There's no easy fix for this, and given the wide FOV of most cams it's really not a big problem. If that's a deal-breaker for someone there are cams which better adapt to that kind of use although you might lose performance or features in using those instead. Since almost everyone goes for central mounting it's hardly something to worry about.
There is no one perfect cam for everyone, but from the looks of things at this point, the A139 form factor should do well for the majority of cam users.
Phil