It would be nice if a MID price was available in general as well as a BID and ASK. It would make sense to add this as an alternative "offer side" even though it is technically not strictly one.

The argument for this being often a good thing to use is that it distinguishes spread widening from a move. When the spread widens, the bid suggests prices are falling and the ask suggests price is rising, and vice versa on spread contraction.

Not a huge issue for many purposes, but this would be a relatively simple refinement with a solid rationale.