Translating the idiom "thinking outside the box" I normally hate overused catchphrases in English, like "paradigm shift" and "out of the box thinking".  But I was recently chatting (in Chinese) with a friend about an earlier situation in which someone had used "out of the box thinking".  (The phrase was used by my daughter's teacher).  I was at a loss to translate the idiom.  Is there some appropriate 成语?  A 成语 would be especially nice because the idiom in English is anecdotally thought to derive from a story of management consultants.  In addition, like many 成语, the meaning is really non-obvious from the words alone.
I think 天马行空 is the one.

天马行空 tiānmǎ-xíngkōng
[be powerful and unstrained like a heavenly steed soaring across the skies]
天神之马来往疾行于空中。比喻思想行为无拘无束。亦形容文笔超逸流畅

but use it carefully,天马行空 does not always commendatory

The imagination of child has no limits like the god horse flying in the heaven , details all show the creative ideas 
孩子的想象力如天马行空，且细节处均见独特之创意。
But the former mayor then delivered a meandering speech that left the audience dispirited
但是前市长讲话却天马行空,让听众失望之极。