Just theoretically, A C wire is just a return for 24v transformer right? So if you were in a situation where you have access to the slab, or just a standard outlet or switch receptacle could you wire nut a wire from ground to C?
A "return" is not the same thing as a "ground". In certain situations it is, but not here. The C wire (together with "the other half of the circuit") forms a circuit through the secondary of the 24V transformer. The primary (at 120V) is a circuit through hot and neutral. While, from a purely theoretical (but not safe!) standpoint, that ground on the primary is "the same as" neutral, and therefore a logical (but not safe!) substitute, that is absolutely not the case for the secondary, which is where you find the C wire.
In addition, let's just say hypothetically that things actually worked that way, which they do NOT. If they did, then if you ever had a neutral/ground fault, conducting many Amps of current at 120V (not 24V!) through the ground system, some of that would end up not only on your 24V transformer (frying it) but also would leak over to the control circuits of your HVAC system and to your thermostat, which would most assuredly not be a good thing.
But the bottom line is that it won't even work at all, because that ground is on the wrong side of your transformer.
Your premise is wrong anyway. Current wants to return to source, not ground.
Ground is not a current return in any respect. A lot of people think it is, either because they come out of low voltage electronics where "GND" is used to describe common... or because they are aware of the equipotential bond between neutral and ground in the main panel.
Also, 24V is an isolated system that has no relationship with ground.