Asuming they will use the same system as now.

For stormblood:

First gear set will have around 255 (higher than Lore but less than Scripture), just as 115 on first HW set (higer than soldiery, less than Poetics).
Poetics was useful till level 55-56, so, probably 270 Scripture gear wil lbe useful till 65-66.
Last Stormblood leveling dungeon will have around item level 285
First crafted gear will have 290, 20 more than Scriture gear (first HW crafted gear was 150, 20 more than poetics).
First end game dungeons will drop item level 300 maybe.
First Tomestones gear will be around 310-320
Now, HW tomes and raid gear jump between tiers are +30.
Second tier gear will be around 340-350 (355 weapon), 330 Omega normal
Third tier gear will be 380-390 (395 weapon), 370 Omega normal
Fourth and last tier wil lbe 420-430 (435 weapon), 410 Omega normal

But. I'm sure, probably, the level jump wil lbe even more than +30 per tier. This is just asuming the same progress as HW gear.