Personally, NDs can be great fun. ND for portrait is something that I have not tried.... But here's something I tried....
ND4+ND4+ND8 and a CPL in the afternoon sun along a normally crowded street, say Wisma Atria stretch along Orchard Road..... you end up with a pix with clear defined buildings.... and an almost empty looking street cos human and vehicle traffic are transient. Kind of odd really cos you know it's not supposed to be devoid of life at that kind of an hour..... there's some minor blur here and there but if you're lucky.... you got an effect...
You can try long exposures via small apertures plus NDs at night as well. If you can get exposure to be like 2 mins, you can wear dark clothing, take a torchlight or a light source and "paint" with light. The effect is kind of amusing.
Well, just 2 other suggestions that perhaps you can toy with. Like NM86 said, there are no rules, as long as u can think out of the box.