That's a tricky requirement because some GNU programs do exactly that: use mixtures of tabs and spaces. For instance, the first three logical tabs are groups of 2 spaces, so 6 spaces. The fourth tab, however, turns into single hard tab.

I think that to do this right, there would have to be a complex option in which you specify the tabbing style. -Widentation=<notation> where <notation> is some way of specifying the style.

There has to be allowance for alignment also. Any line, regardless of indentation method, can terminate in one or more spaces that exist for aligning the fist non-whitespace character with something in the previous line.