Fire Emblem: Three Houses’ Wave 3 and Wave 4 DLC have been detailed in the Three Houses Perfect Guide published by Famitsu, and it seems that it will include Anna as an extra playable character, a sauna feature at the Monastery, dog petting, and more. [Thanks, Serenes Forest!]

Here are the contents according to the guidebook:

Wave 3

Free

New playable unit: Jeritza (certain routes only)

New supports

New activities

New online features

Paid

New playable unit: Anna

New monastery facilities

New online features

Anna and Jeritza dialogue

Around 10 new quests

New activity: Sauna

New costumes: Maid, butler, and more

New Maid and Butler Battalions

Wave 4

Paid

Side story: Abyss Mode

New player characters: Juris, Balthazar, Constanze, Javi (direct translation)

New supports

New online features

New Hero Relics

New classes

New paralogues

New quests

New battalions

New Gambits

New costumes for Byleth (male and female) and Sothis

Fire Emblem: Three Houses is available on Nintendo Switch.