Simple really, do what Hi-Toro did and look at what is around now and make something even more awesome using your own custom silicon and an advanced but cheaper CPU not many people have decided to use
Seriously look how the market has gone...
Apple gave up on esoteric PPC CPU and plumped for cheaper Intel units for performance/costs.
Microsoft & Sony & Nintendo all use ATI/Nvidia GPUs for performance/costs
So I don't think you can have a true Amiga anymore ie a machine which uses different but technologically better hardware to achieve better performance at LESS cost. Dig a little deeper and you will remember the C64 was only possible at that price because Jack Tramiel had the shrewd business acumen to purchase MOS Technologies. So to have that sort of advantage Amiga would need to own Nvidia/ATI and do their own chips at cost price.
So as none of this is possible that just leaves the OS...which actually there are two immediate things that neither OS X or Vista implements in this internet age which are crucial to the smooth running of your OS while it is permanently connected to the web. A new OS will never be a new Amiga though sadly.