thats why I brought the idea with elemental powers up Every element has its very own way of fun - and for your example with slowing it would pretty much work for every element tooWhile Ice (cone slow) and Fire control (targeted-aoe slow/area of denial) had OK slows, my absolute favorite slow came from Earth control. Quicksand had that great targeted-aoe cast that enabled casting from around a corner to mitigate the return alpha-strike while doing double-duty to reduce the DEF of the mobs making them easier to hit for one's teammates. While it didn't slow their attack speed (just movement), Quicksand with a follow-up of Earthquake was my go-to fight-starter for the slow, -DEF (stacking from both Sand and Quake) and -attack frequency from all the mobs flopping around due to Quake.
Almost brings tears to my eyes remembering all those good times....
Water - Ice (freezing your opponent - or friend if you like )
Air - either some kind of storm or compressed air
Earth - Sand slowing you
Fire (ok that one is tricky ) - could imagine a thermic field which cristallizes the dust in the air
(ok I admit, that is not truly fire but a combination of fire, air and earth - but fire isn´t a real element but only the visualization of heat anyway )