It really depends on the number and variation. There's plenty of velvet textures, but not a really good satin. Nesh always is a good idea. Different types of metal and stone. Wood. Wood in panels. Special effects are always welcome. P5 materials combined with props for special effects are also good. I use one of the special effects from The3DWizard on a sphere all the time for an interesting background. There are a ton of free materials over at rDNA, and a few nice ones here. If your materials look like those, everyone will pass beause they've already got the free ones. I included a couple of dozen in my Ichiro Fantasy pack (not very good, but interesting). The pack should be large, and divided into differing types for quick access in the library. There should be lots of them. The pack should be cheap. You'll get more sales that way. Lets face it, P5 materials are underused because most P5 users haven't gotten into them. Those that have may be using Vue or Bryce for final renders and using the materials there. Pity that. The P5 material room is the best part of P5. Enclose a tutorial on P5 material making, and you might just have a winner.