Diadem had a perfectly valid concept, the developers just fudged it up royally. Instead of gatherer's actually being actively involved in Diadem, they just go off and do their own thing...
Instead, those treasure chests gatherer's can find? More common, and should reward various temporary items that go to the party. That way someone gathering has a positive impact on combat, they effectively become a pure support role (Diadem requirement could then be changed to 1Tank/2Healer/4DPS/1Gatherer). Someone is gathering, so you get powerful potions and poisons to use against enemies to speed things up.
Those caves? Make them pretty much the main objective, so people want to explore them. Then, block them up, with either large roots or rocks. Allow them to be destroyed with DPS, but DPS would be very slow. Instead, the roots/rocks would have gathering nodes on them, and if a gatherer clears them, it does massive damage. Then you've got a situation where gatherers are a massive boon in exploration. Root blocking the path? Call in a Botanist.
Throw in a "Gathering causes noise and attracts more monsters" system, and you've got a situation where defending a gatherer gets you more stronger enemies to defeat.
That is how you work gathering into Diadem... SE just put nodes in the zone and figured that would be enough, honestly it is pathetic how little thought they put into Diadem...
As for Hamlet Defense... If it comes back, I really hope it isn't instanced... I want something open world like Besieged... Although really, I'd pass up Hamlet Defense if 4.0 gives us Campaign... Hamlet Defense was XIVs attempt to replicate Besieged with its terrible server. Campaign took Besieged and improved upon it ten fold. We shouldn't settle for another Besieged clone, 4.0 should have some massive campaign against Garlemald with the developers taking Campaign and improving upon that ten fold. Works better for gatherer's and crafters, too. Not a great deal you can gather/make to defend a hamlet, but a full on war? Siege weapons say hi.