A zone of control system would be perfect for CK era warfare. If anything, it fits better there than it ever did in EU4. Wars during that era were often focused on securing the castles that obstructed the way to continue. There are castles on the English/Scottish border that must have been ceded or besieged dozens of times and it was extremely common to have castles in strategic locations that effectively HAD to be captured by invaders in order to advance any further. Wars in CK games should be heavily concentrated on borders, with them only spilling into the heartland if those fall.

This would also, incidentally, better emulate the danger that Vikings posed to most nations. Sailing up rivers or striking fast from the sea made them far, far harder to defend against and allowed conquest of territory that might have otherwise held out.