More like 5.5 than 6 since it's still going to play like 5e

environmental damage is the answer here.

Have your DM introduce a storyline that starts off with interrograting drug dealers on roof tops.

The OneDND version of EK has this at level 7 now as well

He is Vigo! You are like the buzzing of flies to him!

Anus Tart, very nice

They sound like twats

Oh no, it's salt water!

Keep in mind that just because the item is legendary doesn't mean that they get to keep it. After it has served it's purpose it could be destroyed or if they do get to keep it then it could be severely weakened or damaged, or it could even be something that isn't meant for them. Obviously if they don't get to keep it they would still get something out of the creation process.

This gives you some freedom to go absolutely bonkers with it since it's only going to be used for a little bit.

Some personal ideas of mine

1. A Giant Mecha to fight a Kaiju.

2. A dying god needs a new body.

3. A Mastersword like weapon. A powerful weapon that works for a time and then needs to recharge.

4. Are the Warforged a thing in your world? They could literally be the father of an entire race.

5. Are Airships a thing? If no then they could invent that, if yes they can create an upgraded version. This has a benefit of being a boon for the whole party.

6. My personal favorite idea, assuming they're into it. Let them design their own creation, they are the inventor after all.

There's more than 1 way to become a sorcerer

I imagine every now and again an ancient tome speaks to them only for them to roll their eyes and put it in a special chest their parents gave them.

Player 1's father is obviously the Warlord.

Player 2's father is also the Warlord. He killed the father in the same way Darth Vader killed Anakin Skywalker.

Player 3's mother sold their soul for the Warlord to fall in love with her.

Player 4's Father is the Warlord and Player 3's Mother. Except after they got together they settled down for a happily ever after.

This is exactly what I would do if I rolled god like stats

Just go hangout in a whale for a while. Maybe they'll give up.

Cleric's Potent Spellcasting would allow it to double dip on Wisdom. I suspect that's why it's not on their list.

The Barbarian even has advantage on Dex saves!

Not sure it'll come up, but it should be noted that Bronze Dragons are Amphibious, meaning if given the chance they would likely much rather fight from the water

As for magic items, what sort of resources do he have access to? You could easily make custom items that only work for dragons.

It doesn't even work, GWM specifically calls out Melee Weapon

They just serve it to their human buddies

It is