hmmm let me think about this....TIEN! Oh and for my reasons, just read all of my thread "Tien in 1.3?"
http://forum.esforces.com/showthread.php?t=41071
Some other characters you could add are:
#16 would be nice. He is stronger than all the other androids and has more attacks then the generic ones. He can do Rocket Punch, Laser eye, and Hell's Flash. Also, you have make him a better melee fighter by making his attacks deal more damage.
#18 or West Kaioshin for all those who want to see a female fighter in ESF.
We never see either of them do any unique attacks in the series but they do have some potential.
Pikkon is another good choice. Besides the generic attacks, he can do Thunder Flash and Hyper Tornado.
Nappa could also be added to ESF to balance the number of Evil guys vs Good guys. He can do Shougekiha - the attack where you raise your two fingers and it instantly blows up the area around you or the area you are looking at. This attack can easily be made in ESF, just make it that you create Ki Blobs either around you or where your aiming at (of cource it wouldnt hurt the caster). Nappa also can do that mouth attack which I belive is called Eraser Cannon.