I want to chime in that NU will be a playable tier on the SU server once things get around to that point with stats and so forth. Additionally, I agree with the idea that we'll need another split usage tier, given the raw number of Pokemon that we have available to use. NU will be exceptionally large otherwise. I've been mulling over what I'd like to call it, and I really like RarelyUsed (RU) so far. I think it's important for it to have a distinct acronym that differs from the others and doesn't use a word that is repetitive with another tier.
That said, I really like OU/UU/NU as names and don't want to see them changed. I have phased out the idea of "standard" on the server, which was my main gripe before. However, I think the names OU/etc have distinct associated power-levels that people can understand fairly easily. A numbered naming schema wouldn't really have that, since everyone would have to readjust to it. I also think that most importantly the names are a part of Smogon's legacy, and I would hate to change them and abandon that.