What will be the new dominant trend in Design?  

I believe
That with any trend as also the MCM trend the fact that anything goes as long as it is MCM naturally fades and the rare and exceptional pieces are distilled and only become more collectible.

I still think MCM has a long ...
I still think MCM has a long way to go before it becomes unfashionable.
When it does become irrelevant, I think the legacy it leaves behind will be the idea that the average person should have an instantly recognizable statement piece. A sculptural, showy sort of aesthetic. One that focuses on the individual elements that make up a room.