What isn't a "life hack"?
Good question, but has no answer.
Alternatively try to answer:
What isn't "math" and/or "philosophy"?)
(and remove all other forums of SE ;)
Yes the sites you mentioned cover a part of life hacks. However, there is a sort of questions which don't fit on the sites you mentioned.
How to reduce eye strain?
How to easily win an argument?
How to stay awake at night effectively?
...and so on
So the site can contain such interesting questions about very different things in life, which don't fit on another sites.
That's a good question and we don't actually know the answer. I'd strongly suggest looking at the recently closed Life Improvement proposal. Figuring out what would be off-topic would be very useful for the future of this proposal.
What this proposal needs are example question that are bad because they are off-topic. And I don't mean off-topic just because there exists another site that already covers them, that would just turn this in a "everyting that doesn't fit in other sites" site. That is what would help to define the site adequately.
Let me try to provide a better, more discrete answer to this question, one that also hopefully addresses some of the concerns raised above. Again referring to wikipedia, the key concept when it comes to Lifehacks is ingenious, inspired solutions to everyday problems. The distinction here between Lifehacks and Personal Productivity, to put it succinctly, is something along the lines of, Personal Productivity questions are about things you could probably list on your résumé, Lifehacks questions are about things you probably couldn't. So to answer the main question directly: if it's not about an everyday problem, it isn't a “life hack.”
Shouldn't there be a general community for just questions about life that don't immediately fit in one obvious, narrowly-defined SE site? There ought to be a General site, where questions which are off-topic to everything else can be moved to, yes? I mean, it's similar to Personal Productivity, but whereas Personal Productivity is basically only about lifehacks for your life's “output quality,” so to speak, this site would be about lifehacks for your life's “internal quality,” I suppose, or overall quality? Personal Productivity is too narrow of a topic. Or, to put it another way, think of it like this: Parenting is about taking care of your children, Lifehacks is about taking care of yourself.