Yes, seeing as Hoenn is being the only left out region right now, I would say we will more than likely have a few encounters with some of them after the National Dex is obtained. Whether it be in special locations or even through the PokéRadar, I still expect at least a few to be available. Not to mention, along with them being there, more than likely some may be held exclusive like in FRLG had it as well for the selective second generation Pokémon made aquirable.
Besides accessing some of the older areas that were closed before, there will be the new buildings and areas we have seen so far. All of this will give the player something extra to do. Although, adding the extra travel in and a more realistic view point of things will definitely be a nice touch, I agree. It is only to note whether or not Game Freak will implement such an idea at the moment or not.
As for the Game Corner, like in Platinum, HeartGold and SoulSilver for the Euro version will be disabled as well. The reason being is because of new European law that doesn't permit the influence of gambling or anything closely related to such events. So with this being said, HGSS for the Eruo model will have the Game Corner present, but it will not be accessible to play any of the slot machines available within. Although, on the topic of weather, in recent videos dealing with the Arceus event, snow was seen within the game, however most speculate it being placed near the top of Mt. Silver.