Final Fantasy is one of the longest-running gaming franchises to date, with tons of numbered entries, spin-offs, supplemental material, and the like. As such, it's hard not to compare one entry to another. Whether it's in terms of plot details, plot twists, or characters like Clive and Cloud, there's a lot to unpack from the surface level to the hardest. This also means that Final Fantasy 16 will be inheriting this big legacy, indubitably being compared to those that came before it.
Right now, there's very little known about Final Fantasy 16. The main characters have been shown, including Clive, Joshua, and Jill, and some of the politics and world of Valisthea have been shown. Yet, still, a lot remains unsaid. One interesting thing is how the marketing is seemingly glancing over certain aspects, such as Jill's role in the Final Fantasy 16, to divert attention. There's another place that it does this, one that sounds a lot like Final Fantasy 15, and that's the blight.
In the very first Final Fantasy 16 trailer, there is a reference to a Blight in the land. It seems this Blight is pushing the six factions to war as it is a spreading, physical illness or sickness of some sort. The effects thereof are not directly shown, but it should be mentioned that the war, resulting from this Blight of sorts, does seemingly happen. As Final Fantasy 16 seemingly features a time skip, it goes from Joshua and the summoning of the Dark Eikon Ifrit to a War with Eikons that Clive is involved in (and the physical changes to Joshua could hint at the Blight perhaps). That's really all that's known, but that in and of itself has a few implications for a Final Fantasy game.
First off, the Blight is a big issue and will likely be featured in the story; however, it seems likely that this Blight is somehow related to the Dark Eikons or, at least, to the Eikons themselves. As they play a part in the war and Dominants are shunned (or celebrated) based on where they go, there may be some connection between Final Fantasy 16's Joshua, Ifrit's summoning, and the Blight.
Secondly, the idea of the crystal, or Mothercrystal, is very apparent in Final Fantasy 16, and it's likely that this Blight can infect them somehow. Perhaps that's the explanation for the Dark Eikons, perhaps a character is intentionally spreading the blight, or perhaps humanity's relationship with the Mothercrystals leads to it.
Ultimately, it's there and will likely play a big role, but its cause and effects are unclear. It may refer to a general sickness or physical plague infecting people, but the question becomes, then, why that leads to war. On the contrary, some connection, infection, cause, or effect related to the Blight, the Dark Eikons, and the Mothercrystals seem likely. And if all of this sounds somewhat familiar, that's because Final Fantasy 15 features a similar plot point.
In a similar vein to Valisthea's Blight, the setting of Final Fantasy 15, Eos, had the Starscourge. Described as a plague, a mutant strain of Plasmodium malarie, the Starscourge has been for eons. That's one difference, at least it seems, because the Blight appears to be relatively new in FF16, while the Starscourge's history predates FF15 quite a bit. It could be more similar here, but that remains to be seen.