Material. We need more steel slides. Not to say that steel is the best for everything, but it would definitely be nice to have a selection. Beyond that, quality and trades are most important.
I believe that people buying aftermarket metal slides will be the kind of people who value accuracy in terms of recreating real firearms... people who want a metal slide for durability buy a WE or KJW.
Don't know if it's feasible, but maybe slides for rarer GBBs too? There's only one WA M9 metal slide on WGC, and it's sold out. Unfortunately, I don't have any experience with CNC (next year!!) but again, if it's feasible, look into frames, and possibly other parts of the gun. I don't know how much time you're willing to sink into this, but you could probably run a pretty sweet service wherein people mail you parts, and you do a CNC copy of it and mail them back.