Last time they did an advent engine, it was for a battlebox (the 40k eldar vs chaos one) - I imagine it would be like that again
Also last time, it was a springtime battlebox as opposed to a summer release, so I imagine it would be before AoS4. Whether it's a 40k battlebox again (I could see those feathers being for a kroot pech'ra) or an AoS army box for a new faction, who can say