I expect they'll just halve everything, since we're at level 100 now. So rather than the current level brackets, we'd have:

ARR: 1-25
HW: 25-30
SB: 30-35
SHB: 35-40
EW: 40-45
DT: 45-50
New expansion: 50-55, or 50-60, probably better to do the 50-55 for futureproofing

Every Dungeon, Trial, Raid, would have their level requirement halved, and rounded down if there's a decimal point introduced. For example, Sirensong Sea, the first SB dungeon, is currently level 61. It would become a level 30 dungeon with this squish. Sastasha, first dungeon in the game, would be unlocked at level 7. TEA, the SHB Ultimate, would be at level 40. Etc.

This also results in a reduction in 'dead levels' when it comes to Job Action learn rate. There is a TWELVE LEVEL GAP on CNJ/WHM (18-30) where you learn zero new Actions (only a trait at 20 that is a flat 10% increase to damage and healing). With the squish, it'd be a 6 level gap, which is still not ideal, but it at least sounds better. Or they could add something in that gap, ARC/BRD used to have a gap there too, broken up only by semi-useless Role Actions, but they fixed the issue by adding Wide Volley as a lower-rank version of Shadowbite. So they could add something there to WHM, such as... IDK, a new attack spell like Water? Or readd Water from the 15 Class Quest (that's where Fluid Aura used to be, after all) and have it upgrade to Water 2 at level 24, perhaps