Two things, one simple, one nigh-impossible.
1. D&D Lite. It shouldn't be that hard to take the SRD, rip out the complex pieces, put the whole back together to form a simplified set of rules for beginners. I honestly don't know why no one has done this yet.
2. The ultimate DM's electronic toolkit. I use several electronic aids to prepare my games, and still don't have anywhere near everything I need. I especially don't have it all in one place (program). I would attempt to create a single program with the following features:
*Character creation/management
*Spell creation/management
*Item creation/management
*Creature creation/management
*Town/Kingdom creation/management
*Organization creation/management
*Stock npc's
*Campaign organization tools
*Complete copy of the SRD, searchable via hypertext and search-word
*Ability to handle house rules
*Various outputs, including characters, stat-blocks, maps, spell-sheets, combat charts.
I know some of the products currently available have many of these features, but we still don't have one easy-to-use, all-encompassing tool. For example, no one offers a tool that allows me to create characters using all the SRD classes, races, creatures, templates, armor, weapons, magic items, and equipment. And then gets the numbers right. And then allows the easy addition of new classes, races, items, etc. And then outputs the info in the specific format I want. And doesn't require me to be a programmer to do it.
Like I said, nigh-impossible.