Info from that euro gamer interview
* The size of the new area the DLC takes place in is larger than Limgrave and more densely packed than Limgrave
* Dungeons will be a little more intertwined with open world gameplay, rather than the clear divide between dungeon and open world that exists in the base game. The DLC does still have large legacy dungeons
* The expansion adds 8 new types of weapons to the game, 8 new categories had to be created to fit these all the new weapons. There is also new spells.
* There is more than 10 new bosses
* As far as difficulty, the expansion will be similar or slightly more difficult than the end game of Elden Ring, and if you want very challenging content, similar to the infamous Melania fight, yes there are new bosses that are as tough as Melania.
* Yes the new zone for the expansion does include a Poison Swamp, rejoice poison swamp lovers!