Already in the game in the form of crypts and such. This could be expanded upon by having different types and sizes in each biome. Probably already planned to be expanded.
I'm listing it here because what could really add excitement to these, would be that each dungeon type for each biome – could have a chance of spawning a boss chamber if the dungeon is big. The devs could create 1-3 different bosses fitting their home biome, with varied different strengths and weaknesses and strategies to defeat. And these bosses could be randomized in strength and loot table by having 0-5 stars in strength.
2) Big raids.
Big raids added to regular raids. After say , day 100, just as an example. Add the chance of a big raid. A text warning like: Dark forces are gathering, and will arrive in 7 days. This means that for 7 days, no regular raids will spawn. But you will need that time to prepare – because after that time a really big raid will appear, with a boss leading them. This would add a lot of fun and focus on building large castles etc. lategame. Because in the long run, raids won't feel like a real threat anymore. So this could be a way to make all that time building feel really meaningful.
3) World bosses
For each world seed. Add a wandering world boss to ONE random biome for each biome. For example. The dark forest could have a Troll King for example. Mistlands, The Widowmaker, a gigantic spider.
This would mean, that for each world seed. It would be both a challenge, and exciting to explore. Because you would have to find and be able to defeat these world bosses. For each biome you visit, the world boss matching that biome could be there. With appropriate rewards.
In other words, for Meadows there is a unique wandering Meadows boss. But only for ONE random Meadow area in the entire world. Extremely powerful unique non-respawnable world bosses would really (imo) make world exploration exciting and rewarding, and of course, dangerous!
4) "Uber version" of each story boss.
3 trophies to summon the first boss. So what if you, the next time, wish to summon it with 6 trophies? Which would give it +1 star in strength and stats. And maybe 1 new ability per such upgrade up to +5. So if you've defeated the first boss and summon it with +1 star, it now has a permanent lightning damage aura. Or can suck you into melee, or lightning striking the ground around it randomly (just suggestions).
This way you could use bigger and bigger trophy sacrifices to summon stronger and stronger versions of each story boss. Each with a unique reward specific to them.
5) There needs to be really, really long term goals that incentivizes and rewards sticking to a world/character for a very long time for the sake of the games longevity in the long run. Maybe as a toggle, a player can choose "Lethal late game events" to a world seed.
Meaning that after very long periods in a game world. Like 500, 1000, 1500 and 3000 days. Unique events takes place. The last one being Ragnarok happening – where you can stop it.
I'm not THAT versed in norse mythology so I'm not sure how "lore breaking" it would be. But imagine for example… after 500 days you have to defeat the world serpent out at sea. At day 1000, an absolutely massive unique raid against your fortress by either frost giants or fire giants, which you are warned about in advance to prepare.
By designing apocalyptic world events after X amount of very long playing time. If balanced right could really, really make things to look forward to (and dread) coming.
In closing. One of the things that really hooked me on Valheim. Is its potential. The feeling of earned progress. Base building and defense… fuck it I love everything about this game. However – I feel that the survival and basebuilding aspects though great – are still underutilized. Adding more dungeons, dungeon bosses, world bosses around the world (that are unique, with unique loot, that don't respawn and are a real challenge) would really add to the longevity of the game. Lastly, huge world events like I suggested, as a difficulty toggle, could be extremely fun and rewarding for those that intend to sink in extreme amounts of time into a world. Using trophies to summon stronger and stronger versions of the story bosses would be a nice way to recycle and find a use for trophies, that was the main idea there.
Pacing is king.
Short and long term goals and challenges are king.
At some point, most of us just goes bored with games if there is nothing meaningful to do and nothing that challenges you anymore. I feel like these above, could really help Valheim become a game you could sink 500+ hours into.
I have no idea how you guys feel about my suggestions here, or if anyone will even read this. But world bosses, large raids, and enabling lethal large scale scripted world events, could really make this game shine IMO. I'm in particular in a deep desire for long term big story events. Like I said, the last one being Ragnarok occuring. As the ultimate challenge for people who want to risk it all enabling these things.