I wouldn't mind if cars are shared between islands. Heck, I'd be happy if you could open a small menu and change your car mid-cruise while cruising online.
I don't see any downside to including O'ahu again. Options are supposed to be a good thing. What's the alternative? Just have the one island (Ibiza)? Have another island which is much smaller (since upgrading O'ahu is quicker than building a new island)? If O'ahu was so bad would we really still be playing TDU anymore (how quickly did the maps in the last few NFS games get boring?). Sure, the external community and mods help a lot, but I have never, ever played a game as much or for as long as TDU.
I do hope, though, that we are not required to 'discover' all of the roads in O'ahu again in order warp to them via the map. Maybe TDU2 can detect if TDU1 is installed, and import the savegame data from there to figure out what has already been discovered and go from there. Or they could unlock all of the old roads, and leave only the new ones locked. Or, since O'ahu is an unlockable achievement in itself, all of it's roads could be 'discovered' by default already.
I'd be happy for them to keep the old discovery system in Ibiza, of course (which I'm sure they've improved in a few ways).