I think there are a few ways to do this depending on what you want, which is not a 100 percent clear.
If you want your character to walk on an invisible floor beyond the visible one, the simplest way to do it is to add another floor, scale it up and set it to invisible under the rendering options. This will work well enough if your level has boundaries (walls that block your character from going any further).
If you want the invisible floor to continue into infinity, you can set your character’s default land ‘movement mode’ (under the CharacterMovement settings) to ‘Flying’. However this will change the way your character moves and make you unable to jump, so you can do a line trace to only set the ‘movement mode’ to ‘Flying’ when there is no floor.
Putting this in your character blueprint should work (except for when jumping off of the surface):
(the ‘Floor Height’ is a custom float variable with the value set to the z value of your character capsule component’s location when its on the floor)
You could also try to make a custom movement mode if you want other behavior: