Better support for existing materials and lights under new renderers
Having 4 different renderers out of the box isn't very useful if they all use separate subsets of materials and lights. IMO any new renderer should support most standard materials and lights at the very least. If this is not possible, better tools need to be included to aid in the translation of materials and lights appropriate to the various renderers included with 3DSMAX. If you are halfway done a major project and you discover that another renderer would be better or faster, the amount of materials and lighting rework necessary is often a serious disincentive to use it.
I'm envisioning a materials table that would show material names down one side and renderer names across the top. Each slot on the table shows a rendered sample of the material. You could create a separate shader or material for scanline, mental ray, iray, quicksilver, vray, etc but they would all be called the same thing (eg. galvanized metal). The appropriate version would be used according to which renderer you were using. If you originally created the scene with mental ray shaders, only that column of the table would be populated. If you subsequently needed to create iray materials to correspond to them, you could drag and drop them from the mental ray column to the iray column and it would be up to iray renderer to take a guess at an appropriate translation. You could then tweak or replace the material(s) at your discretion.
This would probably run very slowly on anything but the fastest machines, so you would want to be able disable columns or filter by materials in various ways to speed things up. Also, there's probably technical issues regarding running more than one renderer simultaneously (this already shows up in the material editor if you have a mixture of materials types - eg. vray materials render black when you have the mental ray renderer active). Anyways, that would be my ideal material management system considering we won't likely ever be able to ensure compatibility between every material and every renderer.