It depends on how it's pulled off. I'd love a lengthy, flashy fight and ideally as 'Minstrel's Ballad' retelling so we get to fight him as 'intended'.
Extra points if he kills Aymeric because we need a major NPC to actually die and stay dead for Nidhogg to feel like a threat - especially since his 'plan' conveniently involves him staying back and not assaulting Ishgard directly. To be honest I wish they'd have made it so that Nidhogg tainted Ishgard's water supply with dragon blood to turn numerous elezen into crazed dragons who caused chaos from within Ishgard. Yet that didn't/won't happen now.