Aw yeah! Luke, I've made dozens upon dozens of DC customs over the years, so if you need any parts suggestions for specific characters let me know. You can also have some of my old decals if you want, but your designs are much better than mine.
I'd say the most pressing needs would be to finish off teams, specifically the JLA, JSA, and Teen Titans. Firestorm, Zatanna, The Atom, and Elongated Man would all be great choices. For the JSA the Original Green Lantern is a must, and others like Starman, Dr Mid-Nite and Hourman would be great. For the Teen Titans Wonder Girl and Changeling (Beast Boy) should be at the top, as they would more or less complete the original New Teen Titans lineup.
I'm betting that most of us here would prefer Silver/Bronze Age characters, so I don't think you should look at too many modern characters. One pretty easy one to do would be John Constantine, as you'd just need a trenchcoat and cigarette hand. Another good one is Death (from the Sandman series) as you just need the Shriek hairpiece, plus a white head, white arms, and black legs (if you have any goth accessories like those studded belts and wristbands you can use those too).
Ages ago I commishioned Big Barda and Mister Miracle from you, and those would be good choices. For Mister M's cape you used a Doctor Doom one and cut off the upper hood part, so it was just a collar. I thought that was very clever.
I can't wait to see what you come up with!