Well I wouldn't know where to start, but I have a few things I'd like. I liked the charm of the A500/A1200/Other Amigas which were fitted in a keyboard. I'd like to keep that style as much as possible. And backwards compatibility too. That's a good idea.
Then probably to win over the market there'd need to be a 3D acceleration of sorts in it, but it should still be good at 2D. Of course a good bit of processor power and maybe extra sound.
Then as for marketing I'd say do it the M$ way and bribe game companies to suddently change their mind as to where they're going to release a game and have them make a couple of great Amiga only games.