If you're dragging the droppable, as you drag your finger the target could "light up". There are various ways it could light up: it could be outlined, or it's color could change, or it could expand a little, or combinations of these. Note if the drop region expands, then one would need to move a little extra to move out of the expanded region. This would have a slight "locking in" effect, so minute movements wouldn't take you out of the lighted up, expanded, target. I think this would be beneficial. But you have to be careful not to expand a region so much that it completely obscures another (tiny) region, which could render that other tiny region unreachable.
If you're not dragging the droppable, that is you get in a mode in the next touch performs the drop, you could compute if the drop point is ambiguous, that is if the drop point is within distance D of more than one possible target. If the drop point is determined to be ambiguous, the region around the drop point gets magnified, allowing to re-drop it on the magnified area with less ambiguity.