I'm not sure there is much you can do without breaking _something_. Just check the whdload patches to see how many games that expect the blitter to be finished before the cpu gets around to updating the blitter registers. Likewise I'm pretty sure there are programs expecting the blitter not to be finished when writing to it.
The general answer is to clock things faster and have more memory slots free. That would be within the scope of traditional AGA. Next up would be a scandoubler then flickerfixer. If they could manage some heuristics to detect interlace and natively present it as progressive then it would be coolio. If not then a separate monitor driver that adds it to the DBL modes.
For other "added functionality" stuff there has been plenty of discussion over at amigacoding.de.