I wanna see Mattel do a few base buck bodies, but done well. Perfect anatomy, detailed musculature, etc. Put in some excellent articulation. They have the blueprints already in Mafex. I doubt they would do import style joints but at least do the correct joints with the right range of motion. Good butterly joints, boot swivels, toe joints, double ball at the abs and ball jointed waist, etc. Actually sculpt the symbols in their chests, sculpt the gloves and boots on, etc. Give them excellent face sculpts and detailed face printing so the characters actually look good. Extra heads and hands are a must, and a few accessories as well.
It's really not hard to design a line that is good for both collectors and kids alike. With Mattel's budget, it should be easy to get the right people involved and make sure the main collector line is fantastic. If done right it will pay for itself in no time.
But I honestly have seen very little good Mattel products, ever. There's definitely been a few good lines, of course, but it's been few and far between. These need to be essential for every dc collector out there, these need to be so good that everybody wants them. Else it's just gonna be another Spin Master dc line for kids.