What would Buddhists say about 'pantheism'
I think that Buddhists don't assume that there's a "one God". So there's no need to explain the relationship between God and World.

Wikipedia's Pantheism in religion mentions many religions, but not Buddhism. Maybe the concept (of "pantheism") is meaningless when there's no concept of (no meaning attributed to the word) "God".

I suppose the nearest Buddhist analog might be the concept of Buddha-nature.