Just because they are doing a big vehicle doesn't mean they'll all of a sudden start cranking out vehicles from every license. They've only ever done Bat-Vehicles
This was not lost on me. I was merely pointing out that size might not be an obstacle. They keep breaking down the boundaries - no one thought they'd make a 1/6 Tumbler and they did. No one thought they'd make Iron Monger and Cain (Robocop 2) but they have and are. The Bat makes the Tumbler looks small from what we've seen in trailers for TDKR and yet they're seemingly making it. So, just regarding size, the APU doesn't seem as unfeasible as it used to albeit still a bit unfeasible considering its size combined with its complexity and the fact HT maybe no longer holds the license or desires to get it back.