[Propose your ISS extensions here!]

Nauka is simply very, very late (it was supposed to be up there in 2007), and Uzlovoy isn't intended to be part of the ISS; it's the first component of OPSEK.

Uzlovoy is scheduled to be sent to the ISS first, to serve as a support module, and then later to be used in the construction of OPSEK after the ISS is concluded.

[Propose your ISS extensions here!]

Also, the ISS has been extended to 2024. There isn't enough time left in the ISS program to design and launch new modules.

Actually, there are a couple more modules that are scheduled to build onto the ISS, Nauka and Uzlovoy.