Create markers that appear whenever you highlight a possible tile for movement. These markers show if the tile is considered covered by fire, mist, goo, or other environmental hazards. Also make it so that if a soldier’s projected movement path would take them through such a hazardous tile, the tiles in question along their movement line are marked, and the end point they would move to is also marked. This shows that a soldier will subject themselves to these hazards if they take that move.
I suggest this because XCOM 2: War of the Chosen did the same thing, and I found it very helpful. And it wasn’t just for fire, acid, or poison gas; it was also for beneficial, defense-increasing smoke clouds created by smoke grenades, although in the latter case, only the destination showed since only the end tile mattered as that’s where they’d stand to get the smoke bonus. The same could be done to mist tiles in Phoenix Point, as right now it only affects a soldier if they’re standing in it when their turn is done.