Was in the middle of a post on Multiclassing/Hybrid mechanics and thought of some more terrain effects. Some of these terrain effects can get pretty complicated, and in general are things that on the fly I'd only normally think of if a player brought them to my attention. Not wanting to stir the shitpot on "D&D being too anime," but some of these came to mind while watching Fullmetal Alchemist.
Mud: Difficult terrain, and also increases most forced movement effects by 1 (DM discretion, would limit to effects that knock someone around as opposed to taking control of their movement). Mud can be frozen with a cold attack, in which case it only adds forced movement distance, and running might call for an Acrobatics check to avoid falling prone.
Rain: Rainy areas impose an attack penalty to effects with the fire keyword, and creatures gain a +1 bonus on saves against ongoing fire damage. Attacks with the cold keyword also slow a creature for a round if they do not already. Heavy rain might grant a minor amount of fire resistance.
Pillars: I'd talked about pillars before, but I was thinking about allowing characters that can make multiple attacks actually attack pillars in order to knock them on an opponent. Like, slicing through one at such an angle as to cause it to fall on them. This is another option in addition to the normal pushing them over thing and also gives martial characters some more stuff to do with terrain.
Pools: Another one that I'd mentioned before, you can freeze them to slow/immobilize targets. Something else I thought of is hitting them with a fire attack, which creates a zone of fog that grants concealment for a round.
Snow: Grants fire resistance and/or cold vulnerability. Also, could have a staggering kicker effect where an effect that slows instead immobilizes, and if it immobilizes instead restrains.