Actually, NU could theoretically be split into 3, maybe 4 more teirs, of roughly equal size to the current ones.
Which is scary.
This is because of:
1. Vast numbers of fully evolved pokemon. That's over 150.
2: All of the NFE's. Some are viable in the current NU meta, but as you get lower down, more and more will be viable.
This is also the reason where some things are still not viable. When any other water type is around, who'd use Lumeion?
But yes, there could be a split like this, if we ever get enough players:
1: NUbers: NU pokemon by Smogon, Uber in NU. Before it moved up, Aggron would have made this, by my guesses, as a Head Smash destroyed EVERYTHING. Gastrodon and Quagsire were 2HKO'ed, and Gligar couldn't beat RP sets. Hippopotas's sand stream also helped greatly. As it stands, there are a few possible suspects, be it due to any of the three Uber characteristics. (Walrein may qualify, because of it's Stallrein set, and Super Fang, under Defence)
2: NU: Teir 1: This would be the OU of NU. Things like Gastrodon, Medicham, Primape, Haunter and Skuntank.
3: NU: Teir 2: This would be like our UU. Things like Vileplume, Phione, ect, could be here.
4: Forgotten: This is the teir where the rubbish reign superme. The NU of NU! Chatot, Pachirisu, Luvdisk, Farfeched, and a lot of NFE's would be here. Needless to say, this teir would be chaos. A place where Farfeched could actually pull off a Swords Dance Sweep, and Luvdisk's Aqua Jet is the Scizor? Madness.
Of course, for this, we'd need our own server. Can't expct CAP to run four ladders for us, and Doug to keep stats on another three than he already does, can you?