I think the first experience with the game was among the best I had in Fire Emblem, but its also a game that's hard to go back to because the gameplay of each route except Crimson Flower is mostly the same. The Fodlan fatigue only extend to gameplay though, since Hopes showed that there was even more millage to get out of the cast, world and story then we thought.
Its still unclear what the legacy of Three Houses will be within the series. Engage took over practically nothing from Three Houses but was also developed alongside it, so it couldn't have. What lessons the series will take from Three Houses will likely get revealed in the next Fire Emblem.
In terms of story, worldbuilding and characters Three Houses could be a happy accident due to another dev being at the helm, and it being the Awakening style that IS really wants to stick with. Or due the positive reception the next game will take cues from Three Houses in this area. In terms of gameplay I think I would be a little disappointed if the next game does take after Three Houses. It would make the game less unique but also risk bringing a fatigue to the next title. And while I have little interest in ironmanning, I do strongly feel that the gameplay style of Three Houses doesn't really fit with the philosophy that units should be allowed to die, and that this is a shame.