Recently, the game director of DOOM: The Dark Ages held a special stream during which he showed DOOM 3 (dhewm3 Source Port), and also talked about the expansion for DOOM: The Dark Ages.
The game will feature a secret weapon that will take a lot of effort to unlock. The DLC will present more information about the lore.
The gameplay will generally remain the same, but there will be new elements. There will be various challenges.
The expansion will be in the style of Metroidvania: to complete it 100%, you will have to go back (a lot of backtracking).
It was also announced that the Archville (mini-boss from DOOM Eternal) will return in the DOOM: The Dark Ages expansion.