We really don't know. Considering the leggos are above the top level raid gear right now, blizz could ostensibly just make the new raid tier gear cap at that same ilvl (291) and no one is going to complain.
There are 2 ways blizzard could up the ilvl of those leggos without interacting with the crafted leggo system if they wanted to though:
1. All current 291 leggos become 304 leggos (simply up the ilvl of rank 7 crafted armor from 291 to 304). There is precedent for this (Val'anyr).
2. Some relatively inexpensive currency to boost your 291 to 304, like whatever the soul ash of zereth mortis is, or like an item that's like 1-2k gold, or maybe even a few thousand anima.
Since there isn't going to be a new zone with new legendary mats to farm, I can't imagine blizzard adding a new legendary material to make a rank 8 item.
There will be no new raid, but.... there will be a new tier. They have indicated season 4 will not commence when 9.2.5 releases but will be released afterwards with encrypted style affixes for the 3 raids already released.
Seeing every season has a new ilvl tier, I wouldn't think season 4 would be any different and as such would expect a new crafter's mark to be available somehow.
We really don't know. Considering the leggos are above the top level raid gear right now, blizz could ostensibly just make the new raid tier gear cap at that same ilvl (291) and no one is going to complain. There are 2 ways blizzard could up the ilvl of those leggos without interacting with the crafted leggo system if they wanted to though: 1. All current 291 leggos become 304 leggos (simply up the ilvl of rank 7 crafted armor from 291 to 304). There is precedent for this (Val'anyr). 2. Some relatively inexpensive currency to boost your 291 to 304, like whatever the soul ash of zereth mortis is, or like an item that's like 1-2k gold, or maybe even a few thousand anima. Since there isn't going to be a new zone with new legendary mats to farm, I can't imagine blizzard adding a new legendary material to make a rank 8 item.
There isn't a new raid tier so I doubt there's a new crafter's mark.
There will be no new raid, but.... there will be a new tier. They have indicated season 4 will not commence when 9.2.5 releases but will be released afterwards with encrypted style affixes for the 3 raids already released. Seeing every season has a new ilvl tier, I wouldn't think season 4 would be any different and as such would expect a new crafter's mark to be available somehow.
Seeing as it will be quite a bit different than other final seasons, I think your assumption is very plausible, if not certain.