do a couple lions and an eel. if you do this, I strongly suggest not getting two volitans unless you decide on putting them in the 150 ( antenneta, radiata, fu manchu, or any dwarf species is perfect for a 75), a 75 is big enouph for one volitan, but not two for long. also it is better to introduce two lions into a tank at the same time, lions can be very terratorial towards eachother if they're isn't ample room for hideing durring the day. as far as eels go, leopard eel, snowflake eel, and even a chainlink are good candidates for a 75 gal species tank (they're are more, but they cost anywhere between $400-$2000 and require an expert keeper as do sharks and rays, which bring up another bad recomendation that your lfs will probibly tell you).
most lf's with say you can put small sharks and rays ie bamboo sharks, california stingray in a 75 and they will only grow as large as they're enviroment lets them, NO thats wrong the shark will end up getting seasures and deformed limbs, and it will tragically end in death. so unless your going to get a 180 gal (which i think is still too small) i would stay away from sharks and rays.
good luck,
Jake