It's no secret I loooove to speculate so let me present it like this.
If the devs responsible for working on offline franchise, online franchise and superstar were tasked with integrating those career modes together, all the existing features unique to each mode, would be new to the others.
So if all I ever played is one mode, each addition from another mode, is an improvement. For example, integrating player contracts into online franchise from offline franchise is an improvement. Implementing contract negotiations and potentially game play camera views from superstar mode into any franchise mode is an improvement. Being able to access an one player offline franchise through the internet from online franchise is an improvement.
That's what I mean by IF that is the case, "connecting career modes" is tantamount to improving each one.