Since the addition of the new Map, the Mirror timeline has been getting new additions rather quickly! Mirror Magical islands, the Paironormals which are mechanically tied to the timelines, and the Ethereal islets! And they're not going to stop any time soon, the Mirror Map needs to be filled! Aside from some obvious upcoming additions, there are many possibilities to explore and theorize about.
Starting with Zarroë, who is using the Mirror timeline to make all of their dreams come true. The simple fact that the Ethereal Elements are more stable in the Mirror timeline has allowed the creation of the Workshop Ethereals, and recently, the Ethereal islets! We just have to wait for the remaining 4 to release.
But Zarroë isn't going to stop there. The islets are merely preparations for their magnum opus, their biggest project ever, the union of all Ethereals on one island! An Ethereal Gold island, perhaps. And that name might be slightly more accurate than we originally thought! Since the islets are going to be close to their counterpart Mirror Natural islands, it's safe to say the All-Ethereals island will sit in the middle of the Map, in place of Gold island! Zarroë would show off like that.
Another thing to consider are the Primordials. It's weird, since the idea of bringing back the DoF Diamond Packs as actual Monsters has been on the table for so long, now that we have them, I don't know what else can be done with them. I mean sure, there are the 5 Primoridal Elements, but just making hybrids out of them and calling it a day feels so uninspired and wasteful. The Primordials are also limited to one of each variant per islet, so you would have to get them in some other way on the theoretical Primoridal island.
There is a more uncoventional possibility for a Primordial island. They're the Colossals' abandoned Monster designs, so what about abandoned Colossal ELEMENT designs? Just a bunch of wacky, unrestriced Element ideas, like Fungus, Food, Desert, Troll, ORANGE, whatever! Their Element borders would look like the other Primordial Elements. Throw in a Fire Primordial, too, why not.
From wild speculation to something more obvious, the Mirror Fire islands! I will not hear anyone out, we're obviously going to get them some day. As for the design, most predictions base them off of the Electricity Element. Makes sense, considering that it's Fire's counterpart Element. However, the Mirror Magical islands are based on the Outer islands, which throws a wrench in the mix. It would be interesting to see how the "Ethereal incursion" described in the Mirror Natural islands' bio impacted Fire Haven. We need those additional Breeding Structures to zap Epic Fire eggs!
Speaking of the Mirror Magical islands, the future of the Paironormals also seems pretty clear. After the Light and Bone Paironormals release, we'll get some kind of Paironormal island where their Elements combine, just like Magical Sanctum. The question remains how exactly it's going to work. Will we get two islands, a Major and Minor? Will it be one island, but you can flip the song with a button? A weird Schrödringer's cat situation? Whatever it is, I expect that is where the Paironormals are going to end. Just like Seasonals and Mythicals, they get their own island and it's only variants from there.
Finally, to discuss some lore. The Mirror timeline is just a what-if scenario of the Main timeline, meaning that it should have all of its worldbuilding. There are the Mirror Natural islands, so there should be Mirror versions of every Colossal. There should be a Mirror Pocket Dimension, with a Mirror Zarroë. They probably wouldn't be happy with Main Zarroë literally breaking into their turf, right?
There are the Mirror Magical islands, so there should be a Mirror Stair Shaper, with Mirror Magical Nexus. Main Stair Shaper stole the Outer islands from the Prism Gate, and it probably isn't too happy about that, since it has been described as sentient. Is there a Mirror Enchantler? Why wouldn't there be?
There's Wubbox, so should there be a Mirror Wublin island? The Storm exists on the Mirror Map. There are Seasonals, so should there be Mirror Seasonal Shanty? Mirror Dream Crafter and Mirror Cataliszt? The Mirror Higher Plane? Where are all of the dimensions on the Mirror Map anyways??
Yeah this is a massive rabbit hole, but this is what the devs will have to deal with the more they keep adding things to the Mirror Map. Will any more "Mirror-" variants of islands be released, aside from the Mirror Fire islands? Probably not. Will they come up with lore explanations for their absence? Probably not. Perhaps some things are better left forgotten, for now.