I wouldnt want a 1:1 scale HD version.
More like a remake trying to gain its feel, with just a small section.

I also prefer the idea of full expansion, but traversing to the other dimension, rather than just 24man stuff.

I'd also like the expansion to be like 75% FFXI, and 25% some new thing, that connects the worlds, and leads the FFXIV story forward, and away from the expansion once complete.
(patches after would be non vanadiel focused.)

The 24man stuff would focus on some more major endgame elements, like sea and sky.
PotD, would the Nyzul Isle, or w/e its called.

Dungeons are FFXI based, but the trials most likely wouldnt be.

Some zones Id like to see are actually quite bland, and too expansive.
So there would need to be some creativity on how to do them.
(we'd be limited to like 5 major zones with HW/SB zone sizes.)
Unless people dont mind smaller 2.0 zone sizes, with no flying.
Then we could see some more.

Also, the 25% new places, would mean at least 1 zone is in eorzea, not vanadiel, which might have flight, and larger size.

I dont see any real reason to do any of this, other than my bias for wanting it. Anyone who hasnt touched FFXI has no reason to really want it.