i might be remembering wrongly but i think they renamed a 51+ trial by dropping the "(Hard)"
Based on what we know about development plans there don't seem to be any plans to explore Xelphatol at all, which is why they gave it to us in an instanced dungeon.. but there is also no telling how much of the area we would have been able to explore if it were open world. We also will probably have a chance to visit it a second time in Xelphatol (Hard). I also anticipate they may be able to find innovative ways to reuse existing dungeon instances apart from being pure dungeons - All Saints Wake is a seasonal event, but Haunted Manor does set a precedent for that.
Unlike XI I expect new expansions to follow the existing content mold (ie. FATES, dungeons, trials, raids) that XIV has, because XI didn't have a fixed content structure so most content were closed systems and introducing new ones would entail developing new types of content.
That being said HW already introduced a few new types of instanced content - Diadem (which ultimately flopped), Aquapolis (which was decently well received), and Deep Dungeon (which was a huge success). I expect there to be more "off-DF" events like this in 4.0 which may follow in the same vein or be entirely different. The Forbidden Land of Eureka will be one of these, and I hope they pull it off well this time.
Besides that, they also need to introduce meaningful content to the open world so it feels less like a lobby. I've always thought they missed that opportunity in XIV. Popped NMs would be a great addition to the world - they already have the Treasure Hunt system which is essentially that. They'd just need to make the pops more predictable or introduce a tier system and I think it would be pretty interesting.