BEagle:
You're right, there's nothing wrong with the design of the A400M - bar it requiring the most powerful turboprop ever developed, and that numerous "fully compliant" bids for this have later turned out to be too heavy and underpowered.
P&WC reckon their new three-shaft design could generate 15,000 shp, and RR more than 11,000, but it's gone a bit chicken-and-egg: while Airbus could pick one of them now and give the project added impetus, they are unlikely to do so before the project go-ahead, if and when that comes.
Still, if the technological challenge of the A400M is something, what about the Boeing Pelican concept (the wing in ground-effect thing that could carry the equivalent of 17 MBTs) with four props - now there's a design challenge!