Did anyone else propose such a modification to the system? If so, im sorry!
Could it be possible that one might have , say all or some of his euro systems on a single ''My Modular" page and then draw modules from the catalogue beneath, placing them to wichever rack he desires, navigating from one another etc? Maybe a zoom feature to add leverage? It would be a level up for sure, regarding the user workflow!

Multiple systems on one page will not come for several reasons.
But maybe as a workaround you can open two systems in different browser windows and use c und p shortcuts to copy/paste modules from one browser window to the other.

If you have a Unicorn account (WORTH IT!), you can do something along the lines of creating a large-scale "composite" build that encompasses all of your cabs. The Unicorn account breaks you out of the 4 row lock that the free account has, and by setting up various tile rows filled with blanks, and blanks to demarcate the different rack sizes, you can set up a pretty good approximation of what you're describing. And you can also save the modules you have to your "favorites" list so that they'll be on hand when needed.