O.K. I am a little selfish. I want a time machine, and I want it just for me.
Why just for me?
If someone would use a device like this, who knows what trouble could occur on the time axis. Since is known that someone will return in the past is very possible to change something (that is history), and by changing it, everything in the present will different.
Fortunately traveling in time is not possible. So no one could change something that already is done it.