True, I think the closest we'd get is a heavily modified BAT-AT.
To be honest, I don't understand why they made the AT-ACT a larger walker in the first place. Was the AT-AT not awesome enough for Rogue One? I didn't even realize they were different until recently. In the theatre, I assumed they were just AT-ATs with cargo areas. It's a very minor issue I take with Rogue One, but honestly I don't see the two types of walkers on the screen at the same time, so I feel like it's more a marketing thing than anything else. It's like "this walker is MOOOAAARRRRRR AWESOME!!!" Whatever.