I never thought I'd see this, but I saw a Yamaha digital piano once that had a port for a bass pedal controller. It of course had a pipe organ and a Hammond organ voice. I'd like something similar, or maybe the piano itself, though I don't remember the model.
The only other feature I'd like to see is having one or more organ voices and the ability to disable attack. I ask for the cheapest three in case the bottommost isn't so good.