Ciphertext with uneven letter distribution
I would have expected Vigenère and Playfair ciphers to reduce the skew in the letter distribution, so I think you can rule them out for the time being. Here are some other things you could consider:


Perhaps one of the letters corresponds to a space:


  hello world &rarr; itssgxvgksr

Perhaps there is no letter "e" in the plaintext


  A Void by G. Adair &rarr; Q Cgor wn U. Qrqok

Perhaps the text was encrypted using a monoalphabetic cipher with homophones (so that two or more letters in the ciphertext correspond to the same letter in the plaintext)


  hello world &rarr; itxyg vgkxr

Perhaps the text was encrypted with an additional transposition step


  hello world &rarr; itssgvgksr &rarr; isggstsvkr

If the ciphertext contains mixed upper and lower case letters, then perhaps it's just been Base64 encoded:


  hello, world &rarr; aGVsbG8sIHdvcmxk

If the letters are printed using different styles (e.g., different colours or sizes), then perhaps this is a Baconian cipher


  hello &rarr; 00111 00100 01011 01011 01110 &rarr; rrvuv ysljq akjsz amshc wvieh