PvP is first priority so I would not be surprised if future PvE content can only be played using current PvP cards, citing balance as the reason. They have taken the first step with PS4, so phasing out PvE cards should be the next. Siege mode can work very well as a PvP mode, which is most likely why it was even considered in the first place. Retrofitting it for PvE is more trouble than what it is worth.
"Winning with terrormill is not fun? Try losing with it"