Eggplant does one thing well: it melts into the sauce.
If you want that luscious, almost creamy base? Grate a zucchini into the sauce. It disappears and thickens everything.
If you want chunks? Use mushrooms or cauliflower. You choose the experience.
The Golden Rule of Swapping
Don't replace eggplant. Complement its absence.
Ask: "What does my stew need?" Is it body? Flavor? Funky texture?
Then pick from the list. You are the boss of the pot. The eggplant was just an employee.