With the way this game generates composite characters coupled with the sheer quantity of items out there, I'll be borderline shocked if we get any geometry changes to characters beyond maybe the face since that appears to be entirely independent of any equipment. Bumping the vert count on player models overall would be a monumental undertaking this far down the road.
Upscaling textures is going to be the meat of this upgrade IMO, it's a relatively simple job and won't hugely impact performance. My best guess is that skin and hair materials will be the big gains here. Higher quality skin normals should make shading cleaner in strong lighting (Precisely why that was chosen in the preview IMO) and even as a Lala on a 1440p screen, the lack of resolution on the hair textures is immediately obvious.
Where's the PBR thing from? Are you assuming that XVI's advancements are going to get back ported to XIV? Again, that's no small undertaking with significantly more materials being thrown around than I suspect XIV's PS3 friendly roots would allow for without serious work.
Opacity map resolution improvements? That's another way of saying texture uprezzing sir. No 2 for 1s allowed.
Lighting improvements is going to be the other big one IMO. FFXIV's lighting has always had a bit of a miserable dull brown wash over everything if you don't jump straight into reshade to correct it. My suspicion is that we'll see colour correction coupled with some basic reshade style functionality to inject a bit of vibrancy into things when needed. FFXIV genuinely can look pretty stunning when it's not doing it's best to look as brown as Quake 1.
Fog updates, meh, more reshade.
Increased world clutter is interesting. I could be wrong on this one but I don't think FFXIV's clutter is dynamically generated? I'm somewhat confident that it's baked in at creation with the only variable being LOD distances. You can verify this with the Grass Quality option in System Config. It doesn't increase the amount of foliage, it just controls the distance at which it draws independently of the main world geometry LOD toggle. The way FFXIV handles world scatter is very primitive even for it's time and adding more would quite literally require their world builders to go in and add it themselves. Outside of an LOD distance bump this isn't something that's going to change with a graphics update.
As for shadow resolution upgrades, that's another meh. A nice to have for sure, but it's not going to really change that much outside of very specific scenes that use world geometry cast shadows as a visual focus. Remember that your character's primary shadow is already rendered at full resolution, compare that to a secondary shadow from a dynamic light source such as a lamp which is rendered at a much lower quality and that'll show you exactly how far they can go. It'll be a cleaner image for sure, but it's not going to be a significant change outside of the aforementioned niches.
TLDR: IMHO we're going to get higher res textures on common shared key assets like hair and skin, Built in reshade style functionality (Hopefully not just for gpose!) and likely some uncorking of settings such as shadows and LOD. Expecting much beyond that is a recipe for disappointment given how dated this games renderer is. Go equip a Pantheon Fending top and stand in water to soak in those glorious PS3 limitations
Honestly, perhaps the boat has been missed on this one, but the engine improvement I'd love to see the most is making character composition multi threaded and able to scale beyond a single core. The game's servers have gradually gotten better at coping with large world fates etc but character composition is a significant bottleneck on the client end if you've not got a fast CPU. Those huge fate dogpiles have always been great fun, but characters and animations getting culled to keep things smooth feels like an unnecessary cop out IMO.