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What does Zvezda alone lack from being a capable space station?

A recent discussion on the ISS has raised some questions in my mind about Zvezda:

Can the US Part of ISS survive independent of the Russian?

What does Zvezda alone lack from being a capable space station? Are there any existing Russian modules or modules in development that could be attached to enable it to be a capable space station if it is not already? If Russia detached Zvezda from the ISS today, approximately how quickly could they have a functioning independent space station?