This has probably been done before but I want an excuse to talk about a bullet hell boss being added to the game.
Bullet hell boss:
I'd really like to see a bullet hell boss be added. DST combat has been pretty stale for the past couple of years, although boat combat and Crab King have been some nice breaths of fresh air, albeit with their own problems (there's like 4 things to fight in the ocean, one of which doesn't move at all (cookie cutter), and the others are rare spawns (malb and rockjaw/gnarwail), and crab king, well I haven't fought him since waterlogged so idk if the fight is better or not). I'm probably a little biased when I say this, but I think a boss that focuses more on dodging attacks would be an interesting concept in DST. Instead of standard kiting patterns, there would be an aura of sorts around the boss that constantly deals damage so that face tanking while the bullet hell phase is going on is less viable, (this also stops bone armour face tanking) and this aura would deactivate after a couple seconds of bullet hell dodging. Celestial Champion kind of scratches this itch in it's third phase, dodging the laser attack, and to an extent the sleep crystals, is pretty fun, but when you get into melee range it just becomes your standard hit and run style boss.
Unlimited durability weapon:
We already have beefalo which function as not only an infinite weapon, but also as ~cane + mag speed boost, and regenerating armour (aside from ranged attacks). Beefalo however have the downside of 1. they can't be revived and 2. cannot be healed whilst being ridden. These are far compromises considering how early you can tame an ornery beefalo, and how many resources they save. An unlimited durability weapon would not be too strong not only thanks to the existence of beefalo, but by making it a difficult item to acquire, for instance, a drop from the Celestial Champion. Or, if you really wanted to make this item hard to obtain, have it require materials from both the CC and AF. A cool moon/nightmare fuel hybrid sword sounds really cool, maybe it could have a chance to spawn a shadow tentacle and/or gestalt on hit, or it could raise your sanity while held to pair better with the Enlightened Crown.
Critter refresh:
Critters have always been lacking, essentially all being identical and only one of them having any sort of use. First off, I think the hunger mechanic should be discarded, or at the very least remove the begging sounds, it's probably the main reason most people don't get critters. Secondly, each critter should have their own perk. You could say that critters are just meant to be cosmetic, but then why does the lune moth provide an aura of light? A couple ideas could be a slight damage buff with a vargling, the ability to have a broodling cook food (similar to the lavae pet), a small sanity aura when near glomglom (similar to glommer), you get the idea, small things that ideally wouldn't affect gameplay too much, but to still give players a reason to get critters. If players rapidly switching out pets to get the best perk at all times is an issue, then a 10-20 day cooldown could be given to the den after a player adopts a critter.