Why not use a series of attachable bags/containers to connect your main BOB to a Clod Weather BOB Addition or to a Hot Weather BOB Addition. my idea but i need to find a bag i can connect!

The problem I found with bags that attach to each other was they were floppy and sloppy.
I find it easier to have a larger bag I can put the other gear into if I must.
Most of my stuff for winter is bulk insulation. Stuff like extra clothes, parka, sleeping bag and so on.
Summer is not a huge problem here. The only extra stuff for summer might be a floppy hat, bug repellent and spare water.

My preference has always been to have a large bag to fit everything, and then have smaller module bags inside to hold cold weather gear, hot weather gear, etc. Still cinches down tight and stable, and makes it much easier to get to that one thing I need at the bottom without dumping my pack all over the place.

airballrad, I do that with my hiking equipment. I have everything segregated into different color coded sacks: clothing in red, food in green, sleepgear in blue. Makes it a lot easier to make sure everything is in there when I need it. Applying that logic, you could simply pack a warm weather BOB sack, in red, and a cold one, in blue-color coordinated to the season . It may even become fashionable-who knows!!!

My modular approach is for backpacking; when speaking of my BOB, I just use Transformers instead.

...Meaning that I have a Columbia system jacket that can be a rain/wind shell, a fleece jacket, or a winter coat depending how it is assembled. The nylon pants in my BOB can also be shorts, but in the winter they are the layer that goes over the thermal underwear. I keep all this packed year-round so that I can handle a cool night in the summer as well as the unexpected warm January day (70's, very rarely). So I shift strategy to multi-purpose items rather than swapping modules. This and Space Saver bags (zip them shut then roll the air out the valve) make it work.