How much item level we need for the new content?

[ChaseNetwork]

Considering gear drops from the most recent MSQ dungeon are item level 375, I can't imagine more than that.
I somewhat agree with you on the weapon. There's not a good way to get one of those.

Eureka Weapons(Low skill, Supposedly "Play to Win", but also takes less time than building a fresh learning static from O9S)
Tomestone weapons also require Normal Raid drops. (Low-medium skill, Guaranteed within 4 clears and 1k tomestones)
Extreme Primals (Medium skill, Guaranteed within 10 clears)
Savage Raid Weapons (Medium-high skill, luck)
Ultimate Trials (High skill, Guaranteed within 10 clears)

Oh, and Crafted. (No skill. Gib moneys)

Highest casual weapon is 300, and the weapon does impact your damage probably the most significantly. This is one of the more surprising changes from Heavensward to Stormblood that they have made, removing weapons from the level cap dungeons.

For Shadowbringers transition
375 will be the starting point, At Lv.73 is probably where dungeons drop 400, At Lv.76 is where Stormblood 400 gear stops keeping up. If they follow the trends of Heavensward and Stormblood. HQ 409ish crafted gear will probably be mostly kitted out by a combination of the Lv.76 MSQ and sidequests. Of course, if you're doing this around first launch, you're gonna want to take every tiny gear upgrade they give you after Lv.73, cause 3/4 of your dungeon party isn't going to be overgeared for the Lv.79 dungeon like the current iteration will usually be. In other words, people wont be positive about having to partially make up for your gear decisions while also trying to prog a new dungeon that might randomly throw a super mob at you, or a mechanic that only appeared in a Savage fight in the last tier.

[Astrus]

[...]

Highest casual weapon is 300, and the weapon does impact your damage probably the most significantly. This is one of the more surprising changes from Heavensward to Stormblood that they have made, removing weapons from the level cap dungeons.
[...]

Let's not forget the upgraded weapon for Centurio Seals at 340 (I think just doing the weekly B rank for SB and HW gets you enough seals to buy and upgrade the weapon) and the HoH weapon at 365. While not exactly optimal they can be gotten rather casually and are a step above the 300 weapons. In the case of HoH I can't recommend farming just for the weapon but doing it while you're leveling a class in there is a nice bonus.

[Ogulbuk]

Considering gear drops from the most recent MSQ dungeon are item level 375, I can't imagine more than that.
I somewhat agree with you on the weapon. There's not a good way to get one of those.

Eureka Weapons(Low skill, Supposedly "Play to Win", but also takes less time than building a fresh learning static from O9S)
Tomestone weapons also require Normal Raid drops. (Low-medium skill, Guaranteed within 4 clears and 1k tomestones)
Extreme Primals (Medium skill, Guaranteed within 10 clears)
Savage Raid Weapons (Medium-high skill, luck)
Ultimate Trials (High skill, Guaranteed within 10 clears)

Oh, and Crafted. (No skill. Gib moneys)

Highest casual weapon is 300, and the weapon does impact your damage probably the most significantly. This is one of the more surprising changes from Heavensward to Stormblood that they have made, removing weapons from the level cap dungeons.

Very good point. Only reason I have a decent weapon is because the FC made it for me. I didn't even realize until I read your post that there was not going to be a weapon drop from Ghimlyt Dark. I kept hoping for drops for my alt jobs and was thinking I was just being very unlucky.

I wonder if they did it this way to encourage crafting? It's honestly very silly that dungeons dont drop weapons.