I'd heard a reasonably solid rumor (50% chance of being true) that there will be an interim Mk 2.5 before Mk III that is more of an adjustment to existing cards. There'd be an "adjustment deck" with the cards from all the factions that are changed.
For example, PP may decide that Sentry Stones are both weak, and have ludicrously complex rules, and release a new card and rules for it. If true, there's only a few models in each faction where they're so powerful they go in every army no matter what, or are so weak that nobody playing to win would ever use them. It wouldn't cost too much to get the "upgrade pack"
Also, PP seems to have built their business model on copying everything that GW does right (art, background, simple and well edited rules set, interesting factions) while avoiding everything GW does wrong (alienating their customers, arbitrary price hikes, treating retailers like the enemy, faction imbalance) so I wouldn't expect PP to be releasing a Mk III any time soon.