I'm not going to bother asking for thing that DMP probably doesn't have access to, like Shogakukan/Shueisha (Viz) titles or stuff published by Kodansha. Unfortunately, that leaves off a lot of my most favorite mangaka.
Over in the June forums, Setona Mizushiro has been requested a lot. I'd like to request one of her shoujo titles, "Black Rose Alice". It's at 3 volumes so far: http://www.mangaupdates.com/series.html?id=25921 and it's also being published in French. I'd love it if DMP picked this one up. I'd much rather have English copies than French. Honestly, I'd take any Setona Mizushiro, but a lot of her stuff is published by Shogakugan and Viz doesn't seem to want to do anything with it. =(
How about anything by Ai Morinaga? I love her comedy. Duck Prince could use a license rescue. CPM only printed three volumes. I'd gladly rebuy them as CPM's quality was crap.
On the subject of license rescues, Masayuki Takano's "Blood Alone" is excellent. It's in the same loli vampire vein (no pun intended) as Dance in the Vampire Bund, but without all the nudity. I'm not sure if Infinity Studios is officially bankrupt, so this series may be tied up in limbo for awhile. =(
Chika Shiomi. I love her brand of Shoujo-horror and I own most of her stuff in English, but there are couple series left unlicensed that aren't published by the big 3. I would buy these in English too if someone wanted to bring them over.
And, because I saw it mentioned somewhere in the forums below, Hirotaka Kisaragi's "Gate". Most of his work is shounen-ai, but this one is a shoujo. ADV manga managed to print one volume of this three volume series and I really hate to see things unfinished.