a quicksand type of field would make sense, especially if it slowed down the movements of grounded pokemon as most steel types that are grounded don't make much use of high speed, also being sand it could easily block fire, absorb water (water because god damn the number of steel types with a secondary type weak to water) and nullify ground moves due to no secure solid surface, if there was such a field im sure a move of some kind could change it to the ash sand field thing which would turn the tables on steel types taking advantage of the field