Well it seems now a good idea of getting 'odd balls' since they are different.
You could get:
Puffers
Onate Birchir
Eels - would need a bigger tank once they grow
Crayfish (Blue lobster)
Red Clawed Crab
make it a piranha type tank but with no piranhas lol but instead with these vicious predator fish Exodon paradoxus (Bucktooth tetra) - Type in 'Exodon paradoxus' in youtube and you will see how agressive these really are - http

/www.fishdeals.com/characins/tetra/bucktooth_tetra/
These fish are recommended to be in a species only tank and a minimum of 10 in a shoal. These are shoaling type of Tetras but more agressive than Malawis or Piranhas, they will either kill them or get them hiding in the corners of the tank.
Read this for more info. They are really entertaining fish including when they eat.
http

/www.fishdeals.com/characins/tetra/bucktooth_tetra/
The fish plan above - i am not saying to keep them together these are only suggestions for you to think of however, there maybe a few that be could be kept together successfully.
Exodons? you could fit plenty in there
I was typing this down before you put this suggestion on lol, you beat me to it
