estaban fermentando
-they/you were fermenting
Imperfecto progresivo para el sujeto ellos/ellas/ustedes del verbo fermentar.

fermentar

Kibitoshin dio un montón de ideas, con muchos detalles, divertido ante la idea del plan que estaban fermentando.
Kibito Kai gave him many ideas, with several details. He was amused at the thought of the plan they were plotting.
Palabra del día
crédulo