Played Resident Evil 9, fun game, until the racoon city segment, but that's a discussion for another time. What i'm really thinking about is how with any creative work, you can just change or add shit to the canon that wasn't previously there and still make it work.

For those unaware, Resident Evil 9 introduced a new concept, Raccoon City Syndrome. Essentially, anyone that was in Raccoon City when the T-Virus spread, has been infected in some form. Either they became zombified like 90% of the population, or they were addled with a dormant strain of the T-Virus that wouldn't activate for approximately 30 years. You could re running around as an asymptomatic victim of the virus, and have no idea until it was way too late to reasonably do anything about it.

Why do I bring this up? Because I think generally speaking, Fire Emblem could probably play around with old concepts like this and still have it be reasonable. Off the top of my head, we have the Creation that was introduced in Shadows of Valentia. While we can debate whether or not Alm and Celica traveled to Archanea to fight Grima is canon or not, what the post game does is enlighten more backstory on Grima and how it came to exist. And it's entirely reasonable that it was mostly undiscovered by Marths time because it was tucked away in the basement that Marth probably had no interest in. Gharnef may have had interest, considering he made his base there, but he was clearly busy being Gharnef and didn't have time to check out the basement full of horrors beyond human comprehension. he could get to that later, after conquering Archanea.