Preliminary patch notes are up:
Patch 2.5 Notes (Preliminary) | FINAL FANTASY XIV, The Lodestone
Important bits for people who can't read that(can't really copy paste because of shitty website).
They're nerfing the nexus light grind and increasing the alexandrite fate drops, which should make the relic upgrade shit less painful. Books are still gay though.
They're adding a drop to ST/CT, which most likely will be used for crafting.
They're adding new tools(the fuck) for crafting. Named Lucis tool, they'll most likely replace Supras. For gatherers, it seems to involve HQ antumbral/redolent(the shit you farmed for supra). Also the new aetherwheel shit is Alchemy.
There seems to only be one difficulty for Odin.
New PvP map.
New content seems to be: World of Darkness(i90 required), the 3 dungeons(i90 required), Gilgamesh "EX"(i90 required), Odin "EX"(i95 required), Omega Weapon(i90 required). Omega is hidden but the quest requires to defeat the 3 original primals, so it makes sense it'd be something like that rather than something else.
No mention of carbontwine/carboncoat to upgrade items in WoD drops, so might come later.