I believe you can scale the rotation of the bones or something like that using Blend Nodes or Animation Blueprints or something like that. Sorry I have no experience with them but I’ve seen it done. So if you have 1.0 of the animation, it does it fully. If you have 0.5 it only moves half as much, etc.
Epic did an Unreal live training where they fiddled with making the character animation 30% physically simulated and 70% animation driven, so yes there must be a way 
It was funny to watch. The mannequin looked like a drunken crank zombie
Is it possible to have an animation that takes an input to decide where a bone may start and end in the animation?
So if input let’s say 10 you would do a wave at a location and 11 a different location ?
If you read this im sure the response would be why not just make two animations when in reality it would likely need 360 different animations if I went down that route
I’m planning to make a custom sword swing that can be inputted by the player move mouse to the right then left to swing from right to left sorta thing!
Any ideas friends (Ideally blueprints btw)
Im not sure what you mean by giving the player input, but here’s a guess at how you could try to accomplish something like having the player arms(sword animation) follow the mouse cursor.
You can start by looking at animation blending and IK control of your character. The links are some guides from the Unreal site.
With blending, you can have an input number drive the animation. This is closer to making more animations.
With IK you can have a location drive the direction of your character’s bones and define what bones will react. So in theory you get the mouse world position and have the arms(sword swing) follow the mouse cursor left, right, up etc. Basically have the arms setup with IK when swinging and the rest of the body should follow the original animation