I'd like to see DeMar increase his range. But, there's other areas of his offensive repertoire that should see more work.
DeMar has shown that he is excellent at moving off the ball. If he could develop his catch and shoot he could be deadly, especially since he's such a risk to catch and drive.
Also, his handle still needs some work. He's definitely more mature this year. He takes his time, relies on his first step to gain an angle, picks his spots (doesn't run into the help), and is much less turnover prone. But, he still doesn't look great with the ball. He just needs more reps. He's got the right approach, though.
Those two things are more pressing than increasing his range.