Wanting to be different always I have a few suggestions and none are all that advanced, indeed with what you already keep you should be fine with any of them.

1. Black Milk - given you would like an indigo this is pretty close and much less expensive and less clean up.

2. Red Tail Green Rat - Beautiful, just pay the extra money for Captive Bred, at least Captive Hatched. WC and usually full of parasites and even if treated they are pretty mean.

3. Madagascar Cat Eyed Snake - Just look at one, that is enough. They are very docile too.

4. Lucy Rats (Texas or Black) are beautiful, inexpensive and bullet proof from a care stand point.

5. Leopard Rats - Expensive yes, worth the cost, IMHO Yes!

6. Trinkets - One cool snake and dirt cheap (because they can have 5 cluthes a season )

7. Beauties - Be they Blue or Tai or what have you. All rock and most are not tough to work with, they tend not to be the best "handling" snakes though. Mine has never bitten but he is constantly going and now like 8 feet.

8. Trans-Pecos Rat - Highly over looked and very easy to care for and really cool looking.

9. Bairds Rat Snake - see trans-pecos above

10. Black Pine Snake - They get really big, they are really cool and not everyone has one. Oh yea and some of em hiss, impressive but the whole bark is worse then the bite thing applies.

Hope that helps,