I have regular 12-2 wire going to a bathroom fan. Tell me about code issues with just running an extra 12 gauge shielded wire up to the fan so I can do the fan and light separate. Any restrictions, is it supposed to run with the existing cable... whatever.
Doesn't look good.
This is done to ensure that all the wires are kept close together, so that the magnetic fields involved act the way they're supposed to. And also so that the next guy can figure out what's going on, and doesn't have to search a bunch of raceways to locate all the circuit conductors.
If it was a control/signal wire; and was not supplying power for the operation of the fan, that would be a different story.
If the existing wire is the plastic Romex type then you should plan to run another 12-2 with GND wire up to the fan from the switching location. A single insulated conductor is not going to cut the safety mustard. When you run such wire then just common connect the added Neutral and GND wires into those existing connections within the boxes at each end.
It may actually (in some cases) be easier to simply remove the existing 12-2 connection and replace it with a 12-3 connection.
If your existing wiring is in conduit, flex or other shielded connection scheme according to current jurisdiction requirements then you should be adding the additional or replacement wiring in like form.