I believe that there is only one timeline and one future, but we simply don’t know what it is yet.
That “alternate timeline” stuff works only if the traveler can return to his/her starting point and make changes that affect the future. But then we’re left with that old paradox of killing your grandfather, aren’t we?
Try this: You go into the future and learn that your child becomes the equivalent of Hitler or Stalin, a dictator responsible for the deaths of millions. You’re doubly shocked because you don’t even have any children (that you know of). When you return to your origin, you immediately go to a doctor and get either a vasectomy or have your tubes tied, whichever’s appropriate.
You have just changed history, just altered the timeline and violated causality. Unless, of course it’s a child unknown to you who’s already been born, in which case, you’ve accomplished nothing.
Frankly, I think it’s a good thing that we don’t know the future. If we did know, it would ruin the surprise.
Those who do not learn from the past are condemned to relive it. Georges Santayana