The Answer is mostly palette swaps, new enemy stat blocks and like, 6 or 7 story cutscenes all stretched out over a 30 hour dungeon crawler. Not surprised they're pushing that onto DLC, it's the most logical place to put it.
FeMC would take a lot more work than whats probably appropriate for DLC to put together. She has unique CGs, social links, story cutscenes, music and a very different outlook on the story compared to Mr. Dour. It's a shame she's not in the base game but she'd also have been basically a second game to add into the game.