Battletech was released in the mid 80's.
From the beginning you had Natasha Kerensky & hits of SLDF remnants.
The Clans arrived with TRO3050 in 89-90? I forget exactly what month.
I'm going to go ahead & say they had this in mind from day one.
Maybe not the full Genghis Khan feel, Formations of 5, Named for Stellar Bodies, etc etc.
But that they were coming back & that they would be Different & Angry, was likely on the books from Day 1