I'd be perfectly happy with a mail system like oldschool WoW. Mail 1 item(or 1 stack) at a time to a recipient, make it cost something to mail an item, to prevent ridiculous spam, maybe cost an orb of alteration to send an item, and you can either send it normally, or CoD(with your choosing of whatever currency requirements it is).
Would still make it so that people had to actually talk to each other and coordinate a trade, but the hassle of actually getting online at the exact same time would be removed.
I wouldn't want to see a full blown auction house. I would have probably stopped playing 6 months ago if there was an AH in the game, there would be no more need to grind for gear. In D3 I had pretty much the top-end of whatever I could get in like 2 months of playing and then I was done. This game has stayed interesting much longer because of the more challenging grind.