Fire Expansion happened for two reasons:

1. They didn't want to have a majority of the Fire class on Amber.

2. They wanted to make up for the fact Fire and Magicals Island won't get Ethereals, Werdos, or in the Magical Island's case, Mythicals.

Unless we have a scenario similar to this, I doubt another Expansion will occur.

I think it might happen for ethereals or pure magicals given that most of them only appear on one island