So, theres this game called Magicka, and its really cool. So cool, in fact, that you can freeze water and walk over it in the game. Like this: https://www.youtube.com/watch?v=n-qZ7OV2QGU How would you replicate something like this in UE4?
You either replace the water completely with your frozen meshes, or you have invisible meshes slighlty above the water that become visible and collide-able when you use your freeze spell.
Could probably do some pretty neat stuff with hexagon shaped static meshes that are generated as the spell is used. An algorithm to figure out the pattern and then create it should be fairly simple to make and optimize, and it allows similar effects as can be seen in the video.
If you went that route you might as well create the entire field out of hexagons and then you could just update the hexes that are already there.