That´s interesting, something I overlooked. Well with Quests, Sidequests and now Questlines, they have shown that they can think of ways to slightly alternate on mechanics to keep it different and fresh. Might well be that they have something similar prepared for the next set of hero cards
I´m sorry if this has been discussed before and I can not be the only one who thought about this. But given what Blizzard said and how the first to expansions seem to play out, it should be a reasonable assumption that we get hero cards again for the 3rd expansion end of year:
It would fit both thematically as well as from "rotation standpoint". Blizzard said, that this year we will be following the mercenaries on their path and adventures and they will return throughout, having gained more experience each time.
The first expansion, Barrens, was a relatively bland/vanilla expansion. No major new or old mechanics were introduced. We got to know the mercenaries at the start of their endeavor, dipping their little feet into the water so to say... This is shown by them having rather low mana cost and stats. Also their effects are relatively small/specific from an overall view on the game. [note that I am not necessarily talking about the actual in-game powerlevel or viability of the cards, but from a design/thematic standpoint] The cards can be strong obviously, but the mercenaries are still a mere part of your army and not the center of it. Wailing Caverns Miniset then was their first raid if you will.
With Stormwind things get spiced up. We have one of the major mechanics, Quest(line)s, return. And through that we get to see the mercs again,even following their path in-game. The actual reward cards themselves show us more experienced and lvled up characters. They have higher cost, higher stats and their effects can be huge, to the point where it will outright win you the game on spot (priest). The mercs have grown to the point where they now ARE the center of your army/deck and you build around them.
But what could top that of in the last expansion? It would only be fitting, for the mercenaries to have gathered enough power and experience over the year, that now they become leaders of the army themselves and are able to replace "you" in form of new hero-cards. It would fit thematically and the last time we saw hero cards was galakrond, which at that time will be far enough in the past for Blizzard to reintroduce the mechanic.