I assume the relic will only let you skip the initial bullshit quest or something, but not much more. It makes no sense to require to do the old relic for new players/anyone switching to the 3 new classes, and while they could simplify the quest a lot to make it faster, it still makes no sense to force people into lvl 50 content before they can get a weapon that's better than some vendor white garbage.
Like for example taking the current first step relic, I would say it'd let you skip the go in that zone and find a shitty white item, find a WP drop, craft a weapon, meld 2 materias, kill 8mobs of each type, kill the 3primals, get 175soldiery(or however many it is nowadays), run AK. Not in that order but basically remove all the meh stuff. The only stuff left would be fight Hydra/Chimera, which are unique fights only for the relic, so it'd make sense to have to do these to upgrade the relic. The primals could be part of that too, kinda depends. So in the expansion, having a finished relic would let you skip all the dungeon/crafting/buying/running around/shitty quest stuff and only fight the big guys for your upgrade. Plus a unique title and I assume they'd add it to the Salvage guy so you can Glamour unique skins that only old relic owners can get(the blinding shiny Nexus for example or the dyable Zodiac).
The weapon was almost always behind raiding content rewards anyway, so it was never meant to be the most powerful weapon in the game and as such they don't really have to reward people who did it that much, other than cosmetics shit.
Gear reset is a given, but the extent of the reset is important. I'm kinda expecting a soft reset, as in dungeon gear to be 130 or MAYBE 140, meaning it picks off right where Final Coil gear is. This way current raiders can skip a part of the bullshit farming(still will want to farm better drops, or if there's new stats/stats are reworked, whatever is better now) and go straight into raiding, while new players only have to run dungeons for a few weeks to be caught up. Also doesn't completely trivialize every content before. Entry quest rewards at i90 or i100 or so, so any casual player who picks up the game a bit before the expansion has adequate gear to keep leveling right away. They could massively reset though and new gear be like 170 or whatever then all your raiding gear is garbage by lvl 55, but I don't think so.
I expect stats rework, especially skillspeed, maybe a new secondary stat to offer more variety. Materia rework most likely, though who knows. XP in the old world reworked(more xp for everything, a lot more xp for FATEs/Leves/Dungeons) to speed up leveling 30-50%, removes the "holes" in the quest path too for new people and even let them skip stuff. New classes potentially having their own starting areas, or Au Ra having their own starting areas. New player experience bonus maybe, like you buy Heavensward you get an upgraded moogle cap which increases xp from 1-40 by 50% or whatnot.
Bunch of shit is probably changing though, and we don't really know nearly enough. I think they'll be revealing a lot more shit at PAX East, since that's the last big conference before the release afaik, most of the other stuff is in the middle/end of summer.