IGA seems to want to avoid using the same game mechanics from one game to another as much as possible. SotN -> HoD, lost the sword, got the whip, new weapons were 'attachments' and redid the entire spell system to use subs. HoD->AoS/DoS, lost the whip, back to sword, everything is your spells now, collect em all, 3 character party J mode. AoS/DoS -> PoR, drop the everything gives you magic, now less do, but more items, whips return, partner system. PoR-OoE, drop the partners, drop the endless weapons, return to just a few like HoD, mixed with limited AoS souls.
IGA seems to avoid the "I got a whip, and sub weapons" and "CV3 team" whenever possible. This makes me think he's avoiding the 1999 game based on this, aside from it being the climax chapter.