It has been awhile since I have used UE4. Thought about a ball game to get back in to it.
Can you adjust how a ball will accelerate depending on how steep the land is. If so, will it automatically start rolling backwards if you do not have enough momentum?
The physics in the engine already handles that automagically. Or you do not wish to use physics and do things manually?
Apart from that, you can line trace directly down and get angle of the floor, map range that value into acceleration that accumulates in a variable. Use the variable to add to world movement.
I believe the character movement component (?) has a couple of method of checking the floor below you, too.