This implies that the overall train traffic gets increased a lot, so the quality of intersections and the train network as a whole stops being just theoretical problem
uh, it definitely is not a theoretical problem now, with megabases people are building.
Also does that mean that the expansion will require significantly bigger production than vanilla, or are those (and belt/inserter) changes for sake of the mods ?
The production is definetly expected to be bigger. I'm not sure how much bigger because it very much depends on your personal goals. If you decide to make almost everything legendary in the very endgame (like I did), the sheer amount of production you need for everything is huge, and the factory becomes a monster (and I enjoy it that way, obviously). I have almost 10kspm base, and yet, the science part of the factory is quite small compared to the other things.