Hmmm, let's fantasise about my dream house ...
Open plan v. separate rooms - open plan any time as a general rule.
If I had to knock 2 rooms together, it would be the living room and dining room, NOT the kitchen and dining room. It is sufficient for the kitchen to have a small breakfasting area, but not the whole shebang of 8+ diner.
Reason: I hate people in my kitchen when I am cooking. People get in my way, standing in front of the stove, the fridge, the sink etc when I am dealing with hot liquids and heavy pans. Also, the mess and smells - we do A LOT of heavy duty cooking in my house, including curries and roasts. It is not always a pretty sight.
If it is just the family, we can just use the breakfasting area. But for entertainment, there is a big dining room cum living room to open up - it will have patio doors leading into the garden, of course
Agree that formal standalone dining rooms are old fashioned and hardly used IMO.