they may replace the battle frontier with some thing newer. They do not do frontier in games any more
They didn't implement the Battle Frontier specifically in Unova, and they haven't yet implemented it in Kalos, but that doesn't mean that the Battle Frontier won't make a return. After all, Heart Gold and Soul Silver had elements of Crystal that weren't present in the original Gold and Silver, and Hoenn was the birthplace of the original Battle Frontier. I don't find it likely that they would remove one of the more popular draws of the Hoenn region, even if Ruby and Sapphire already had the Battle Tower, unless they do go through with Delta Emerald as a follow up, which actually
is unprecedented.
They will do precisely what they've done for three generations straight now.
When an ORAS cart is battling an X/Y cart, in XY the Mega-evolved Pokémon will be displayed as the Pokémon's normal form (such as has been done with Deoxys, Shaymin or the Genies in the past). The Mega Evolution animation might be played, but the model itself wouldn't change.
I don't understand why people don't get this. This non-dilemma has existed for ten years already, and it has been solved flawlessly in the past. Why do people think it'll be such a big problem now?
That may indeed be what happens, but it has also been a thing to have the Friend Codes unique to the game, and not the system. They may have been able to get around the issue of introducing "new" Pokemon and items mid-generation in the past, but that doesn't mean that what they did was the best solution, it just means that it wasn't as feasible to patch. Unlike the Berry Glitch, which
needed to be patched, this issue just made the game play go a bit less smoothly if the games weren't "compatible". Technology is advancing, and the needed patches are much easier to implement, either online, or via game, similarly to the implementation of the Berry Glitch Patch.
Yes, Generation V had interaction with the Internet. More-so than Generation IV, and certainly more than Generation III. However, Black, White, Black 2, and White 2 were made for the Nintendo DS, not for the Nintendo 3DS. The cartridges themselves didn't have much capacity, if any, in the way of being able to be patched. X and Y, as well as Omega Ruby and Alpha Sapphire have access to SD cards to hold their patches for them.
Just because it worked in the past, doesn't mean that it was for the best, and it certainly wasn't flawless.