Is there any reason why the campaigns should be played after each other? For me it looks like it would be better if all four of them would be independent from each other, if I don't overlook something.
There was a reason in the past when the barbarian campaign was used as tutorial. But this reason has gone imho. So I really would vote for having all of them visible but only the first scenario of each campaign playable. the next scenarios should be either invisible or greyed out.