I dunno what the price of sourcing all of the GB and GBA pokemon games would be, and I never managed to pick up a half decent GBA flash cart at a reliable shop (I don't ebay, which probably affects that greatly), but I did get an R4 for my DS for comparison fairly easily, if you wanted to go down that route.
Edit: Regading the SP shape, they next did the micro where the point was to make it cheap and tiny, but after that came the original DS which had a very similar hinge design to the SP. Of course, they had to add flanges to accommodate the controls which had been pushed out due to the second screen, and the undercarriage is a lot bigger largely because of the bigger screen pushing the battery out to the side, but in terms of it's contstruction at least it's very reminiscent of the SP. The DS Lite struck out in a new more modern form factor and pretty much lost the link back to the SP though.