Mid-Towers would be the only reasonable candidates for something like this as laptops just were never intended to be majorly upgraded and iMacs have case dimensions that are too radically different to put any standard parts in them. There are so many variations on the Intel motherboard design that I'm certain there's at least one or two designs that already are 98% compliant with motherboard requirements that these machines would have. At most you're talking about turning a port sideways or moving it an inch to the left or something trivial like that which can be fabricated very easily. PowerMac G3s/PowerMac 9600s use the same case, all G4 mid-towers use the same case and G5s us the same case as Mac Pros, so no alteration there is necessary. Apple can only help themselves by offering a PC price competitive desktop solution regardless of what form it enters the market.
While all of you guys are blowing hot air about how ridiculous an idea this is had you even stopped to consider how much cheaper it would be for Apple to produce a motherboard package for an already existing Midtower than to design, build, promote and deliver a new MidTower? Certainly Apple has stepped away from the Midtower business, but I personally know of quite a few publishers, newspaper/magazine that have moved copy editors away from Macs because Mac Pros and iMacs were prohibitively expensive and Mac minis were not expandable making them to feel insecure about buying reasonably priced future proof hardware. Apple has grown in other areas, but these are places that were all PC not that long ago and could switch back at the drop of a dime. (or at the expiration of a lease)
The problem is that you dont grasp basic hardware design. What your suggesting isnt in any way cheaper than a new Mac. Your understanding of the cost of moving components is not only wrong, but incredibly wrong. What were telling you is that your idea is just plain stupid, from economic, time and support perspectives. There is no "drop in" solution, because creating one costs a hellva lot more than you seem able to comprehend. Lets look at the problems you fail to notice:
1) The PSU in G3s and G4s isnt powerful enough to drive modern CPUs and modern motherboards at any reasonable CPU speed and use modern graphics, expansion cards. (For instance I have a 980W ( I think, it might be more than that, its certainly not less) PSU in my expandable Mac Pro to let it power modern graphics and expansion cards, my G4 MDD has a 400W PSU).
2) Half the ports have changed, and youd need a different motherboard for each iteration of G3/G4 because of differences in motherboard mouting, size and port placement, so theyd need to make 6 boards (G3 Beige, G3/G4 PCI, G4 AGP, G4 DA, G4QS and G4 MDD)
3) G4s cases are all very different, they even differ in the number and location of PCI slots between models, as well as cooling solutions, so youd need a different board per model as already mentioned (MDD has its PCI slots at the top, whereas a G4 AGP has them at the bottom, the AGP also has 1 less PCI than the DA/QS/MDD models). The G5 and Mac Pros cases are incredibly different inside - they dont have the same layout, at all, due to the massively different cooling requirements of the G5 and the Intel Xeon chips - heck the Mac Pros internal layout has changed between the Mac Pro 1,1 and Mac Pro 5,1. (G5 has HDDs in the top and PSU on the bottom, with the CPUs sat on top of the PSU, Mac Pros have the HDDs in the middle, PSU at the top and CPUs right at the bottom) - and the G3 and 9600 dont have the same case at all - the G3 has 3 PCI slots, and the 9600 has 6 of them - massive difference. Just because they look similar on the outside doesnt mean they are in anyway related on the inside.
What Im saying is that its not hot air what we have attempted to tell you, its the truth, and if you dont like it, well tough, because there is no way of altering it to fit some fantasy of Apple offering upgrades to Intel CPUs for 5 - 14/15 year old machines.