I've been playing the game for about a week or two. So note that some of these things could be in the game and I just didn't experience them or know about them.
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1. Make power-ups more visually clear as to what it is. If one power-up is blue and one is lightish-blue I have no idea which is which unless I hover over them which puts you out of the action mentally. I would suggest maybe making the symbols different for powerups that deal more with hit points (like fire or poison) and powerups that are status effects (like sleep or freeze).
2. Make power-ups droppable/tradable. If my friend is about to die it'd be nice to give them my pokeball.. I mean.. the health power-up.
3. Make health upgrades droppable. You would probably do this by clicking on a health bar and then a small popup comes up asking if you're sure. What I mean is you'd be able to drop the health upgrades you find after you use them. You, of course, wouldn't be able to drop health past your starting default health. This would help when someone joins your party mid-game and you have like 10 life and they only have 5. The problem with the current health system is it requires your team to be fully aware of everyone's health, and that doesn't always happen in a pug.
4. Make a chat channel that allows you to speak to everyone in your world or whatever it's called. What I mean is you know how there's different towns and they all have their own arcade levels? Make it so if, for instance, you're in the first town you can talk to anyone that's in the town AND anyone that's in any of the levels associated with the town. That way if a level is too hard you can ask people to join you. It'd also be nice if I could talk to my friends easier without having to whisper all the time.
5. After you say something in chat remove focus from it automatically. Can't tell you how many times I've said something in a level and then tried to move with wasd and ended up going like "awwswwww" into chat.
6. There are some objects that take 3 or so hits to break. It'd be nice if stronger, more heavy weapons didn't require the 3 hits. A good way to go about this would be to put each weapon into a class. There could be a "heavy", "medium", and "light" class. Then do something like give a block 100 hit points. Then make it so no matter what kind of sword you use in that class it still takes the same hit. What I mean is make it so that like ANY light weapon would do 25 damage a hit (so it would take 4), ANY medium weapon would do 34 damage a hit (would take 3 like normal), and ANY heavy weapon would do 50 damage a hit. Also by putting the weapons into classes you could create new equipment that give buffs for specific weapon classes. For instance you could have a 'Medium Damage Bonus: High' effect that would buff the damage of all medium class weapons.
7. Instead of hitting bombs with the nerf bat, instead make it so they are plantable without charging but the blast radius starts out small and increases as you charge. This would essentially mean you could plant bombs to attack single enemies without having to charge but to attack multiple enemies with a bomb you would need to charge the bomb.
8. Make it so you can preview what an item looks like before you buy the item/recipe.
9. Make it so spikes do damage to enemies. I've noticed some things seem to hit enemies (like poison vines) but I've never really seen any monster get hurt by spikes except for the giant turtles. It could add a strategic layer to hit monsters into spikes with weapons that have heavy knockback (like the troika). You could even do things to flesh out the system like make ghost creatures immune to spike damage (since they're incorporeal). You would have to improve the enemy AI though as they all seem to just walk through spikes as if they don't exist.
10. Make it so there's some sort of timer delay (maybe 5 seconds?) where you can't self-rez after you die. Self-rezzing seems to increase the cost for everyone so using it should be a last resort and too many times on pugs I've seen people just use it right after they die before I have a chance to run over and rez them.
11. If you get booted or leave a level you should get refunded energy depending on how long you were in the level. I entered this one level and there was a team. The leader booted me instantly and I lost 10 energy.
12. Make some sort of pet system. It doesn't have to be a physical pet that goes with you in levels. It could be something as simple as a slot in your character screen and you get a passive buff depending on what type of pet you have. Something I noticed is you end up with more materials than you can shake a stick at and there's nothing to do with them. Maybe you could make it so that the pets need food periodically or else the slot locks and the passive bonus is gone until you feed them again. Then make it so you can use alchemy to make pet food. This would make it so that there's a reason to get materials outside of normal equipment.
13. Make it so you can switch equipment in "non-combat" levels in dungeons (like the one with the merchant and replenishable hearts).
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That's all I can think of for now. I'm sure there's something I'm forgetting but that looks good.
2: You can already drop them, just drag them out of the slot with your mouse.