If you want something "similar" to a BP, as in a python, that can comfortably live in a tub, but is just a bit "more", look into Angolan Pythons. They are getting easier to find all the time and prices have come down dramatically. Still somewhat rare, get bigger then BP's, are more outgoing and usually don't have that feeding drama that Balls may go through.

You could also look into Boas, Hog Island Boas don't get huge and are a fun snake to have around. Red Tail Boas are also great, but can get quite bigger, esp. females.

If you want to go to Colubrids, some Rat and Bull snakes can get pretty big and are fun to have around.

Russian Ratsnakes are especially cool imho. They can be kept in a display cage, they like to watch what you're doing. They are also game for anything, real "buddies".
They start out rather plain looking, but look gorgeous as adults. They get to a pretty nice size and aren't as "squirrely" as some colubrids. Also, no heat needed unless your house is really cold. As a matter of fact, they cannot be kept to hot, or they get uncomfortable.

Thats just a couple of ideas