I know IPACS is working on the A350-1000 right now, but it would be great to have the A400M as the next completely new plane. Of course IPACS should take their time to add the shorter versions of the A350 and the 787.
The main argument would be that it would be the first military transport aircraft and the third recent military aircraft. Also, it would be the second big turboprop aircraft after the Dash-8. I know working on the new plane takes a few months.
But especially because of this I would say the A400 would be the perfect new next plane, because it combines so many things that the game is lacking of in case of aircraft variety right now.
(Keep in mind this is no critic because I think IPACS is doing a great job in general and it takes a lot of time to develope aircraft's)