What would be a good tank mate for my Bearded Dragon?

I have a Juvenile/Adult Beardie that is very well behaved and living in 20 long and about to move to a 55. I was thinking with that with the added room he might be able to have a tank mate. A divider is possible. So just about anything will be good. I was thinking a uromastyx would be sweet. Let me know what yo guys think and if you know something that would not require a divider that would be even more awesomer!!