Zvezda has crew accommodations, orbital control, docking ports to expand (forward end) and accept refueling (aft end). The docking compartment can be used as an airlock for spacewalks. It is far more complete on its own than any other module in the ISS.
Fundamentally, Zvezda is Mir-2. It is the actual Mir-2 module. Had ISS not occurred, the plan was to use this module to start a new station complex.
Like Mir, Zvezda doesn't have Control Moment Gyroscopes (they arrived on Kvant-1). Some work needs to be done to improve communication abilities: Luch-5 satellite relay infrastructure could be utilized more heavily.
Where Zvezda lacks compared to Mir, is in the forward docking compartment, it only has 3 (or 4 if you count the one attached to Zvezda itself). Forward, zenith and nadir. No side ports. This is only an issue as a standalone, as the UM for the future Russian segment is a docking module with 6 ports.
There is not a lot of room in just the one module, nor a lot of power available, which is why Mir had the various add on modules. But by itself it could probably do a good job as a space station (as many Salyut stations and Mir did before it).
As it happens the Russian plan is to use the Nauka MLM module, due to be launched and docked to the earth facing (nadir) port on Zvezda, along with the UM docking ball (6 ports, so lots of options), and the Solar Power platform to form the core of a new station.