Thanks to everyone that replied!!! My fiance is the one that really wants to start the aggressive tank and can't make up his mind on whether or not to do it. I was thinking a sfe as well, but I wasn't sure if it would be comfortable with how big it would get in a 55 gallon, but if it would be possible and it would be happy (as happy as it can be in captivity), then that would be a really great. Also, a puffer was another possibility, but from what some of you have said, they won't get along with the dwarf lion, and the dwarf lion is a must according to my fiance.
If you guys or anyone else have any other suggestions, that would be great, keep 'em comin'.
p.s. The Great White would be a nice addition...