Writing is never a waste of time, and if you love something enough to write fan fiction about it, I say go for it. Don't stifle your creativity. As for the premise, that's the beauty of fanfiction, it can explore concepts that the source material didn't.
At the end of the day, your creative outlet is your own. If you want to write it for your own pleasure than do so, if you want to write for an audience in mind, then perhaps do a bit of research and read what is already out there on the themes that interest you?
Also- if you want people here to read your stuff, there is an excellent critique thread in this forum as well as lots of critique +Groups
That depends on what your objectives are. If you want to publish this, then basing it off a movie may not be a good idea. If you just want something fun to do, then hey, go for it!
You don't need our or anyone else's permission to do something you feel may be enjoyable. If it makes you a better writer in the long run, then you've even derived some practical value from the experience.