The foot location and the space needed for the front wheel movement are a problem when attempting to get the seat as low as posible. The lower the seat the higher the feet are above the riders bum. I don't know how uncomfortable this can be for long rides because all the body weight of the rider is consintrated on the bum. The solution would seem to be a small front wheel or a very long wheel base. If the feet are moved forward to be beside the forks they need to be raised to avoid the front tire movement.
The area in front of the rider can/should be filled up as much as posible for aerodynamic reasons. The problem in this area is the space needed for steering...rider arms & handle bar movement.
The space behind the rider is constrained by the rear wheel/suspension movement. An interesting design aproach would be to start at the rear with the cargo space/boat tail and once you have the ...4 bags of groceries... accomidated you then move forward to the rider and seat height that will put the riders body in front of the the cargo space/boat tail. You may find that there is no need to lower the seat to the minimum.
