I love this game! It's beautiful, fun, and exciting. Even better with friends. That said, I feel like there's a lot of room to improve (which makes sense, its in early access and run by like 7 people). Here are a few features I really want to see added to the game.
Magic: I think the combat is fun, but unrefined. I would love to see some more ways to do combat. Imagine a craftable wand/staff from some odd drops that progress like the other weapons. Magic could use stamina as a resource, and vary from combat ranged magic to spells that buff and protect allies or fear enemies.
World shaping: Getting a flat bit of land situated just by the coast to build that perfect castle can be a struggle. I would love to see the ability to edit and modify the terrain faster, either in a sandbox mode (which I know they announced as on the way) or in a regular game. Or, alternatively, some building options to create a foundation (concrete foundation from Fallout4 comes to mind for this) to build a flat surface.
"Rewarding" system: Hard to explain, but farming, breeding, etc., don't feel reward like they do in other games. In Minecraft, both of these options yield XP, which is important for other things. Valheim just kinda gives you literally what you sow; this feels a bit unrewarding. Translating XP to VH doesn't make much sense because of the skill system. Maybe a skill related to breeding/farming could be added? And to sailing, too?
Combat progression: Killing enemies should feel rewarding, too. Maybe accumulating trophies and sacrificing them at the starting alter (or hidden alters throughout the world?) increases the odds of spawning 1* or 2* enemies, or increases your damage against them, or increases drop rates, difficulty, or maybe unlocks skills/perks/traits that benefit your character in the game in some way (decreased stamina use, increase food duration, something that is beneficial to your character, but doesn't break the game).
Travel: Travel by boat can feel a little boring. Aside from the occasional sea monster, nothing happens. Travel should feel dangerous in Valheim (imo), with the seas being ruled by fierce animals. Maybe allow for ship/cart upgrades that make the vehicle more tanky, or faster on paved roads, or give it more storage space.
Lighting: i hate resign and want a magical lightbulb to light my house. Surtling cores? Yellow mushrooms? Greydwarf eyes? Please… I just want to not have to add resin all the time.
Of course, these are only my unrefined ideas. I want to hear what features you all want to see, and how they could be balanced in the game!