I totally would love to try out an African House snake. I've heard they are really good snakes and a lot of fun.
As to the question about whether or not to get another Corn or expand into other areas, I say this: Get what you feel comfortable with.

It is always fun to expand, though. I started out with a Corn. Then one became three. And three became five. I finally had to ban myself from getting any more. LOL!
If you want a different colubrid, though, maybe try out some Milk snakes or Kings. I don't have a King, myself, but I do have a Sinaloan Milk and she is just the most darling thing. Granted she is very small (I got her as a hatchling), but she is curious, friendly, FUNNY (she does the silliest things) and above all, just as interesting (to me) as my Corns.
Also, if you wanted to branch out into the boa area, but don't want to go too big, you should check into Rosy or Sand boas. My Rosy boa is wonderful. She's a very slow moving snake and she's curious. The thing I like most about her is that when it is time to be handled, she doesn't fight with you....and when it is time to go back into the enclosure, she slithers off to her hide without a struggle. Most of my Corns HATE going back into their enclosures, so it's a ton of fun to get them back in there sometimes.
No matter what you choose, I'm sure you'll love it. I have found that it is always so much fun getting a new snake.