Similarity in absolute value of nearby DFT coefficients
Because the basis vectors for adjacent DFT bins are absolutely orthogonal, the values in the adjacent DFT result bins can be completely unrelated, and thus their ratio can be anything, depending on the data.

If you use a non-rectangular window function, then there will be some leakage of spectral peaks into adjacent bins due to convolution with the transform of the window.  But this depends on the window chosen.