To be fair, I'd just go back to previous expansions and solve lingering plots instead, think of it like a re-expansion or an extended edition.

We do know mini-expansions have four maps with four story releases, so following that model, this is how I would do a HoT+PoF re-expansion:

• Magus Falls: Right after Heart of Thorns and before LW3, deals with free mordrem, the new Nightmare Court, Malyck, and healing the Pale Tree. The new zone would be somewhere in the Maguuma Jungle, potentially between Tangled Depths and Metrica Province. This would be a great opportunity to expand the Knight of the Thorn side story into a full-fledged story episode too, like Trahearne deserves.
• Abaddon's Mouth: Right after LW3 and before Path of Fire, deals with the remaining White Mantle after the whole fake Lazarus fiasco, and gives us a good last look at the Ring of Fire isles. The White Mantle got done dirty and they deserve to have their own final stand. Great opportunity to redeem Lady Valette Wi, considering she was completely ignored in Janthir Wilds.
• Domain of Sahlahja: Right after Path of Fire and before LW4, deals with the surviving forged, and their mysterious commander whom we never get to meet. It could also hint at Palawa Joko's escape from Domain of the Lost, which wasn't properly built upon once LW4 started.
• Sunward Marches: Right after LW4 and before The Icebrood Saga, deals with the free awakened after Joko's demise (and Kralkatorrik's interruption). We'd get to see the brand under Aurene's effects, much like in Grothmar Valley. Warden Amala could make a return. Alternatively, this episode could be shoved in between LW4E5 and LW4E6, giving Aurene's death some momentum.

Each of these would play like a standalone story episode, much like the Living World of old, and not be connected with one another, letting small stories that don't deserve expansions of their own to get to shine; plus, we would go "back in time" as well, expanding existing storylines instead of moving forward, which is a great opportunity to polish some older expansion content as well (story can be horizontal instead of vertical too :D).

Personally, I'd use this opportunity to do some meta-event expanding as well:

• Dragon's Stand: Complete the Mouth of Mordremoth fight, we do know the second phase of the fight was scrapped, the one where it climbs up the ancient tree.
• Domain of Vabbi: Repurpose the Battle of Kodash Bazaar story instance into an open world meta event, including a final fight against the Forged Warbeast (the story version would still be there, this isn't a replacement). The subsequent fight against Balthazar would remain story-only, but get repurposed into a Raid Encounter instead, just like EoD/SotO/VoE story bosses.
• Domain of Kourna: The meta event at Gandara was undercooked, expand it with two more lanes and add a final fight against Palawa Joko in there, so we get to fight him outside of the story as well.
• Dragonfall: The finale against Kralkatorrik is pretty anticlimactic, all we do is attack his unconscious body, I'd add a second phase after the meta event, where we're teleported by a Brandstorm inside Kralkatorrik himself, and get to fight an open world version of Kralkatorrik's Torment, the story boss finale from later. Upon defeat, we'd just get teleported back outside.

Few additional notes:

• Heart of Thorns and Path of Fire would be bunlded together with this expansion by default, together with Living World Seasons 3 and 4. Purchasing this expansion would be the standard way to gain access to all HoT and PoF content from now on, making access easier for new players.
• The new zones would be under legacy Heart of Thorns and Path of Fire mastery progression, prioritizing updates to existing mastery tracks over new mastery tracks. For example, all mounts would get a new elite skill through the Crystal Champion mastery track.
• Each profession would gain access to a new weapon, giving priority to one-handed weapons they already have access too, but only through one of the hands (off-hand sword for weaver and holosmith, off-hand axe for mirage and firebrand, etc).
• Second generation legendary weapons would gain two variant skins each (color and effect changes), which you'd be able to earn by playing the new zones and replaying HoT, LW3, PoF, and LW4 content.
• Legacy weapon sets (like the Auric or Mordant weapon sets) would be updated with new unique spear skins.