I'm already distinguishing blockwise positional elimination from row- or column-wise positional elimination; that's what separates my Trivial and Basic difficulty levels.

However, I agree this could be done better by paying attention to the number of empty cells. Basic difficulty level also introduces numeric elimination (observing that all but one of the possible numbers in a given square are ruled out, which will probably be due to a combination of row, column and block clues); but this is the obvious method of deduction used to fill in the last number in a block, a row, or a column, and that's definitely something that ought to be allowed in Trivial level. (Currently, in any puzzle labelled Trivial, there's always a blockwise-positional-elimination way to fill in any such single empty cell :-)

So perhaps I should rejig Trivial versus Basic so that all those modes of deduction are permitted in both levels, but it checks how quantitatively complex the deductions are in each case. Hmmm.