Regarding the "add-ons", I do like the 100 and 135mm finder settings on the Canon bottom loaders. Of course their quality is marginal by modern standards, but I actually find them useful and have used the 135mm focal length on a Canon quite a bit with good results. The common wisdom is that 135mm is too long a focal length for a rangefinder, but I disagree.
The X-sync flash ability with a standard PC flash connector as seen, for instance, on some of the Leotax bottom loaders is also handy for practical use. I find this much more straightforward to use than the weird flash sync control dial on the Leica IIIf for example. I'm not really keeping track, but I think the Japanese cameras by-in-large also did a better job of integrating the viewfinder and rangefinder functionality of this basic design.
Of course added capability can impede long term reliability, or require more frequent servicing - Leica's simplicity does have it's advantages!