I think there has been a lot of behind the scenes stuff in action going on with the servers (not much of a stretch as this has been flat out indicated by the devs). I think this has been making strides to free up resources on the servers so that some additional options can come in to play, as well as some interesting systems that can realize a few new effects. So, I'm speculating by a long shot here, but let's dive into the meat of this tease (which I personally think is kind of the second half of the poem, as the first half feels like prose indicating that big change is on the way):
"With vast open waters and secrets below,"
With this part I believe it is an allusion to an expansion of the underwater part of the world. Probably some interesting stuff in regards to the Merfolk as well as some fun lore exploring their relationship with both the Pirates of the past (Pirate Lord, Flameheart, Graymarrow, etc.) and the Ancients (which we will touch on more later here). I'm expecting some evolution and interaction with some of these underwater structures and ruins, and the inclusion of hostile Merfolk that were hinted at in the art.
"the shroud that’s alive see it ebb and flow."
This is where I see some of the slick stuff coming into play. The obvious assumption here is that we are getting a Map Expansion, but I think that has more to do with the next line in the poem than this one. My hope here is that they will be using some of their slick tech that can allow for changes to happen at off times outside of updates, that perhaps the Shroud will potentially actually shift around more regularly as described in the lore. This could allow it to consume areas we have gotten used to and expose areas we have never seen. This could happen potentially more regularly and not explicitly require an active update to accomplish if they can leave things in the world but mostly not render them when covered in the Shroud.
Perhaps they will have sorted out ways for is to better visualize the Shroud as well in the live game. Perhaps something like during the Maiden Voyage when we make our way through the Shroud and we can see the ominous wall of clouds with lightning raging withing as opposed to a red filter being applied only when we encounter it in game.
"The horizon is vast will continue to expand,"
This is the part that I think really points to us seeing new lands coming up in the future, as it is essentially explicitly about that whereas the previous line is really more about the Shroud itself. What could these lands be? Could be a lot of things, people have made many different suggestions regarding what they might want to see with this, and no doubt the devs have had ideas about this for a long time internally.
"And fruition will show this was never our land."
For me, this sounds like we might be finding more out about the Ancients somewhat more explicitly, the ones who were here on the Sea of Thieves even before the Pirate Lord and all us Pirates. So, a big old expansion on the lore of the world that will likely directly tie into much of the above stuff (as the line sort of feels like it ties the rest of the section together in the closing bit). This seems like it would be a great spot for some more Tall Tale action to come into play and really dive into that potential mass of lore that I suspect is waiting for us.
Again, I'm wildly speculating here, but all of that seems like plausible outcomes for what was hinted at here. And I'd be very happy with a lot of that too, all of it really. But I don't know all the secret stuff the Partners do, so take all of that with a grain of salt.