Some random Ideas

[Katarak]

Alright, let me start off by saying that the current support items are already insanely overpowered. If anything, the way would be to make strong cades, tomes, barrels etc. legendary items, since they're easily powerful enough to justify that.
I mean, you place three barrels and can kill all of w15 rag, the supposedly strongest wave in the game and in case somebody didn't survive that experience for some reason, you can just use a tome and revive everyone on the server, with the only downside being a 20% chance for a pretty weak enemy to spawn (and a 10% chance to spawn three), which can be easily avoided if you take a little bit of care. On top of that, the fact you can just tome again while there's wraiths alive, makes it almost impossible to fail. therefore, something that buffs that tome even more is not needed; the only valid change to that would be a nerf of some kind.
About the potions: the equipment we have is already overpowered as is, same here: don't need a buff of some kind, but much rather a nerf. Awakened gear is breaking the game, no need to make it even worse. Also, there are some items already existing in the game that might interest you:
Heraldic Standard:
(eq, ":var0", "itm_heraldic_standard"),
(set_result_string, "@This banner symbolizes your status as champion, and causes allies to fight with greater fervor.^All allies within 8 meters of you gain a 20% raw damage bonus as long as this is on your person. ^Cannot be picked up from the ground. "),
Caltrops:
(eq, ":var0", "itm_ni_caltrops"),
(set_result_string, "@These spiky metal objects can be thrown onto the ground. ^Anyone who walks on them will be staggered and damaged for 5 hitpoints. ^Has 1000 hitpoints, destroyable, Non-refillable."),
There's also a Smith Hammer as well as a script to repair cades using that hammer in the game files.
itm_smith_hammer
With what I already said, I don't think I need to explain why an even better barrel is a bad idea.
An unbreakable cade is ridiculous, since you only need one cade to completely block out all bots on half the maps in the current rotation.
There's already way too much gold present in the game with phoenix feathers, relic fragments and orichalcum dropping from bosses pretty regularly. Making 2 million gold a possible drop would just make things worse.
That "lottery" is literally just gambling, straight up casino.
I'm well aware that the fact that there's only a 50% chance (or even less) for a bot to spawn in a wave can indeed fuck up your quest progression, but long-term, it should be more or less balanced. At least we know for a fact that those wave spawns are indeed random, unlike the loot system.
I like all the other ideas you have for quests, except for the Support Item and Donation Quests.
I absolutely love the idea of what you call a Nation System, but there should also be something that gives every single player a common cause, like "Kill Odin 100 times and the drop chance increases by 0,1%". That'd have to be one-time and the loot system would have to work properly for that to be implemented of course, but I think you get the idea. 
Investing legendary essence into your character is insanely broken. Imagine taking 40 essence to give your character insane stats, like 500 two-handed proficiency AND 63 strength.
IF there were a way to craft legendary items, I think logically it should be double the distilling value, so a pride would cost 12 essence, volcanic daggers would be 8 etc etc. It would certainly make sets easier to obtain and actually make every loot feel worth it, since you're not dependent on looting an exact item, but instead you can save up to craft one you actually need. On top of that, it would fix the ridiculous prices of things like volcanic daggers, since there's now a maximum price for every legendary item (at least in essences, the price of those can still be funny).
"Ultra Endgame Crafting" is probably a bad idea, since the result would be an even more broken weapon I imagine. Apart from that, the price for that is kinda low, it would only be the 100 essences.
About the Challenge Mode, in addition to what venom said: bots on rag already deal 25% more damage than if they spawned on hard or normal.

Hey, thanks a lot for the detailed feedback. I get where you’re coming from, and that’s exactly why I mentioned that these ideas aren’t balanced yet. My main intention was simply to bring more variety to support items. I thought it could be a fun way to introduce more legendary options and different playstyles. They wouldn’t necessarily need to be stronger — they could stay on the same power level. For example: if a normal barrel deals 500 damage, a Stink Barrel could deal 50 damage over 10 seconds instead.
And I agree that nerfs could be part of the solution. Limiting players to one tome and one barrel per run could already prevent a lot of the current abuse.
Regarding potions: I don’t think they would have a huge impact. Something like a 5‑second buff with +5% damage would be more of a small utility tool than a real power spike. Also you would need to throw it on the ally and if you miss and hit an enemy he gets it instead.
About the existing items you mentioned — they sound great, I just haven’t encountered them yet, but I’d love to see them in action.
For the character‑investment idea: the reason I suggested a limit is exactly to avoid the kind of broken scaling you described. Something like a maximum of 5 points total (1 point = 1 attribute or +10 skill) would keep things under control. You could even restrict it further, like 3 points total and only 1 point to attribute and 2 to skills. There are many ways to keep it balanced.
Legendary crafting could also be scaled however needed — doubling, tripling, or more. I just like the idea of making loot feel more meaningful and giving players a way to work toward specific items.
As for the ultra‑endgame crafting: I get your point. My thought was simply to give players who already have every hero and every legendary upgrade something extra to work toward.