Just thinking about possibilities
Level 1: Speed -5', or set to 3/4 if higher than 30'
Level 2: No stat bonuses from Str, Dex, Con. Penalties still apply
Level 3: Speed set to 1/2
Level 4: No stat bonuses from Wis, Int, Chr. Penalties still apply
Level 5: Max HP - 1/4
Level 6: Disadvantage on...