How do by-room electrical code requirements work for a house with open plan or nontraditional rooms?
There are no requirements for GFCI receptacles anywhere in any house.  Certain locations need to be GFCI protected, but you can do that with as little as one GFCI breaker if you are committed. 

The bathroom must be a separate physical room, Or at the very least, the WC.  You might be able to put the shower in open space, but the users of the shower won't appreciate that none too much! 

However what is relevant to the electrical codes is the areas which are near water appliances, including the WC and shower, but most particularly, the bathroom sink(s) which might be in open space.  Whatever area you designate for the bathroom sink must have receptacles nearby, on a quasi-dedicated circuit according to bathroom rules. 

Code will require that your design include areas to prepare food ("kitchen countertops"). Regardless of your design intent, any "counter" areas where a person reasonably might prepare food, must have receptacles such that a machine anywhere along that counterspace only needs a 2' cord to reach a receptacle.  That bookends to a law that requires 2' cords on food prep appliances. It's not required for places you wouldn't be able to put an appliance.  

The kitchen countertop receptacles must be powered by at least 2 circuits of 20A, that are dedicated to receptacles that you can justify as being in the kitchen area.  The bathrom receptacle circuit must also be 20A, and all must be GFCI protected.  However, this is not enough in practical terms. The heat applianaces typically plugged in at kitchens (coffeemaker, grill, popcorn popper, toaster) or bathrooms (hair dryer, curler) are almost universally 1500 watts.  Two of them will not fit on a 20A (2400W) circuit.   

So you are really better off installing considerably more circuits, at least 2 in the bathroom and 3+ in the kitchen.  You ought to be able to toast the bread for your George Foreman grill burger while making coffee. Your house has 48,000 watts of service, it ought to be able to support 4,500 watts of appliances. rolls eyes 

The extra cost of splitting 1 circuit into 2 is piff, $60 ($50 GFCI/AFCI breaker, $10 cable).