What kind of fish do you like? Schooling fish or what? School of giant danios would be good, some pictus cats or loaches (not clowns, smaller ones), maybe a smaller bichir like a senegal or delhezi?
My jack dempsey is about 4" and he fine with my 7-8" oscar and my 2" meeki but anything else i put in my tank seems to be killed by mine. I am doing the same setup as you are with getting a larger tank but the Jack seems to want to be territorial with new fish in the tank, best bet from what i have been told is to put the new fish first in the bigger tank then put the jack in so he is not territorial.