lol Ozzy, you call that a wall of text? Wait till you see mine

Endgame wise: Albion is a terrible example, because it has the same issue haven does. Lots of interest early, and then the population slowly drops. Outside the early game, people get bored quite quickly.
The only games that ever achieved lasting stability as sandbox builders are Starwars Galaxies and Ultima Online. Sandbox minus the building, the only one I'm aware of is EVE.
Starwars Galaxies fostered forced socialization via game mechanics, and long term made crafting high end items an issue of RNG hunting and patience. It was easy to craft ok gear, but truly excellent gear tooks months. The crafting system was intricate and complex, but once fully mapped by front runners guides made it easy to navigate. Limited PvP outside formal wars if I remember correctly, but faction warfare (empire vs rebel) was a common end game activity.
Ultima Online, I actually can't attest for ultima never having really gotten into it. I just know that it had very good longevity and was primarily sandbox. Player housing, crafting, long term skill grind were all components. PvP was full loot, and a frequent component of the game.
EVE, forced socialization via game scope, but really at its heart eve has two components. "Indy" players who like to make stuff, and then pvpers who like to blow stuff up. PvP exists at many levels, from small group to large scale war. It's very difficult to do pvp solo, but in 3+ man groups it's quite common. The important note about eves faction based PvP is that it maintains itself via scope. The rewards for owning territory are large, and the area controlled is incredibly vast. Ergo, it's impossible to dominate all of eve space, manpower becomes a significant issue. Indy types are too common, and the game always clamours for more people to pilot ships. Even indy players can engage in pvp due to how the games market system works, so going head to head with players is nearly a universal concept.
The biggest takeaway I get from this is frankly that PvP is a meaningful driver of endgame content. "Emergent" content as I hear it called, and haven has some of that. I honestly feel though that pvp has too heavy a weight to make it "fun" for the average place in Haven due to the permadeath system (I'm going to lump in base permadeath, but mainly due to quality competitiveness). So you get a small kernal of PvPers who enjoy that, and then the rest of the less hardcore pvpers get shied away after a death. Meanwhile Haven's crafting is quite engaging for crafting types, but often there's a disconnect between crafters and pvpers causing schisms in cohesion. Namely resource competition (both crafters and warriors need fairly similar resources to progress), difficulty valuing pvp importance for a village (primarily outside large factions, for smaller groups where warriors usually get killed without having good impact), and morality issues (due to the weight of pvp in this game, many crafters are morally opposed to offensive actions).
So what happens is that the main people left playing the game are hermits enjoying the trading/crafting side of the game trying to avoid being noticed, the rare crafter who likes supplying large factions, and the rare PvPer who also doesn't mind intense grind and likes risking weeks of progress in fights that last at most a couple hours. For minor factions one of the biggest issues I see is the ability to either fall behind the quality/stat curve and becoming out competed, or frankly getting killed off. Some people have mentioned sieging is too difficult between factions right now so the second issue might not be so huge. Even outside character permadeath, base permadeath exists as well. So factions can truly be wiped off the map for a world if a proper breach happens. Quality grind wise, the game feels very much like a job due to how it works.
So the pvp in haven is niche, and anyone interested in endgame pve content risks falling behind if they aren't grinding every day. Some quality grind even requires bots or not sleeping like a sane human (carrots>mulch>wood quality chain). This quality grind is compounded by character or base pvp permanently putting you out of the running.
Fixing pvp (outside the actual system, as in making it appeal to a larger audience) is... well a debate in haven. I've argued many times over my viewpoint, and frankly other people have raised it as well. Nothing new to add to that conversation.
Fixing pve (and its indirect impact on pvp), I've never really thought of it before but since seeing realm bonuses I wondered if maybe the one thing we're missing is some kind of catch up mechanic. Bonuses to help the general playerbase catch up to the leading faction. More could be done as well to make livestock less punishing for keeping up their quality as right now I hear a lot of farmers complaining about the volume of grind behind them to maintain a high quality herd.
Edit: Please don't suggest splitting up havens already small community. If we're going to experiment with new rules, well it is eternal alpha. It could be set in place with conditions like "This will only last this world, pending review and player feedback".