I'm not sure a development studio would risk sinking the type of budget required for GTAIV into a Subsim, but I take your point about the possibility and tech.
There is of course nothing stopping you from taking something like the SDK for CryEngine3 and using it as a free platform for subsim development. The advantage there is you could experiment/demo with it for free with the option of moving to a licensed model with capital investment or leaving it as a community project. Time and effort being the limiting factors...
We'll see soon enough if SH:Online has substance.