Hey there! I have a 75gallon too, and they are too small for bala sharks.
As was mentioned, an African cichlid tank, or Latin American cichlid tank would be great. An oscar would be quite happy in your tank (however, I wouldn't put more than one in there).
An oddball tank would be a lot of fun. I believe the only eel good for a 75G is the Peacock Spiney Eel. Tire tracks and fires grow WAY too big.
Umm... a school of PSE's, a large school of tiger barbs (these things are bursting with character, although they aren't oddballs), and a few smallish plecos would be kool. Bristlenose, clown, and tiger are a few pleco species that would do well. I think bulldogs (rubbernoses) would too.
There are SO many things to pick from. I think the best thing to do is to browse the fish index here until you see something you like. If it's the right size for your tank and compatable with the other fish you'll put in there, then get it. But I do understand how hard it is to decide
Good luck!
Oh, and to answer your question, I would strongly advise against the 2 balas with a Dempsey. Balas really need to be kept in groups (and are too big anyway), and with only two, they would most likely be bullied by the Dempsey. That cichlid didn't earn the name of a legendary boxer for nothing!
You could, maybe, keep a couple of Dempseys, firemouths, and other large, robust cichlids together. I would check with the cichlid forum just to be sure.