There are tons of different fish you can put in a 75gal tank. By tropical do you mean you only want "standard community" fish?
To narrow it down, what are you looking for? Do you want several schools of small fish with a larger center piece fish or two? Are you wanting several larger fish? Are there any fish you are particularly interested in?
How about a bio-tope tank, you could go with Asian, Amazon, New World or African Cichlids, research what area you are interested in and then get the decor/plants and fish found in that area.