Well, if you might have an idea for a 'new' platform. Why not start your own company developing them. Ignore all fellows saying your stupid, that was told to Jay Miner is those days too.
Custom chips is the way to go, when you want to do something different form the main crap. Without those custom chips, it will just be 'another' PPC(?) platform like all others.
Capable of doing just the same like the other, you need a new, fresh, innovating ideas.
You will also need to rewrite kickstart, but that had to be done anyway. Kickstart is unix based, so start from there with Amiga in mind.
The name 'Amiga' is just a name, maybe possible to buy the Amiga Inc. is dead..
I though many times of doing something like this, but haven't had the guts to do so...
Oh, and don't let anyone scare you by saying custom chips is very expensive. They are not, there are a lot of asic specialized company's, that can make you custom chip for a few thousand dollars. The reason company's like Sony and Microsoft spent so much money is because of the overpaid employees, which think they are so important.