The problem is, as with any cheese, and butter too, the salt encourages the fluid (in this case, whey) to leave the curds. So if you don't use salt, the end product is less solid, more watery. Unless anyone knows another technique...?
As to alternative flavourings, I haven't made halloumi but with other 'virgin cheeses', the standard options include garlic, chives, paprika, chilli -or any other kind of, including cracked black - peppers, any green herbs (sage, parsley, thyme, oregano / marjoram, etc.) I don't know which would work best with halloumi but would be interested to hear the results of your experiments!