They're more likely to solder the graphics chips than they are the CPUs. Most people would be more interested in upgradeable CPUs anyway (i.e. most consumers, not you or me). I honestly think something like a mini with an MXM is the most we could possibly hope for from Apple.
Not going to happen. Ultra320 SCSI is Pro stuff - Buy a Pro machine instead of wanting a consumer machine to do the job for you (that'll be Apple's view). FW800? Not a chance. ESATA? The Mac Pro doesn't even have that. No way.
Edit: Logan, you're original post said "But it seems to me that, the people who are asking for it, are asking for too much. They basically want the same expandability as a Mac Pro but cheaper. No my idea is: Don't make it expandable. Make it upgradeable.". Anything with PCIe slots is expandable and asking for FW800 and PCI slots is asking too much imo.
What if that PCI Slot was filled with USB ports, and if you removed it you wouldn't have anymore USB ports. People who would want to put something in that slot would have to sacrifice those USB ports.