The SP campaign was cut from the initial release, although we will likely release it in some form of post-release content. But yeah, you pretty much described what we had planned, LastSpartan.
The SP Campaign was written as a prequel to the events in the Online Kingdom mode and does have heroes work together and travel the world. Although it was mostly focused on Prince Erian and civil war between Menthorn and Dagbor and the orders of wizards during the War Without Kings and didn't have a whole lot of interaction with the other races.
The Online Kingdom/MMORTS mode is likely to be available offline with some minor changes, but I can't promise that. However, the unannounced game mode our team is currently working on is exclusively for the SP audience.