They could, but  they didn't update X/Y for the new Megas introduced in OR/AS. If this is Gen 6.5 (I really don't think it is) then the 6.5 gen game (Z or whatever it is) will be the only which has it. 

Pokemon generations last from 3-4 years on average, that means that it wouldn't actually be out of the normal for a new gen to come in November of this year.

People are saying that the idea of Gen VII coming now goes against established franchise traditions (a third version of the main games), but so did Gen V. Also in saying that, there is absolutely no precedent for a single new Pokemon being introduced in a third version game, ever. Forms yes, actual Pokemon, no. 

I'm 95% sure we're getting Gen VII this year. X and Y left a lot to be desired story and content-wise, but there is little reason to continue them with a third title when a new gen would sell more and earn more for Nintendo/Game Freak, especially given that they broke that tradition last gen anyway. 

The problem with a 7th gen this year is the wild card: the NX. If the 7th gen is indeed coming (and Magiania isn't just another megaevolution of Diance) it will have to be a NX launch title, because there's no way Nintendo is going to pass up the oportunity of boosting the NX hardware sales with a Pokemon game. The 3DS won't have two Pokemon gens. Only the GB and the DS had them, and both of them sold more than 100m units, so it made sense to capitalize on that massive userbase.

It's nearly impossible for it to be a 6 gen pokemon (hackers would have found the code, like they did with diancie, hoopa and volcanion) and there's no such thing as a half gen pokemon. only cahnce for it to be 6th gen is through dlc but game freak never tried to add pokemon to a gen after the games were completed. it's probably a 7 gen pokemon showing up early, like togepi, marill, kecleon, lucario...

It's not like that anymore. Look at mega evolution. You could say it's a form but still. And TPC can always surprise us, they have been breaking the norm for awhile now. 

Mega evolution doesn't change the pokedex. GF never added pokemon to a gen after it released.

BW came out just months before the 3ds. Gamefreak never seems to have new pokemon gens as launch titles for nintendo's handhelds. So don't say it's certain when it never happened.

Again, by the time time BW came to the market, the DS was well above the 100m units sold. That was a time when handhelds were starting to see some pressure from smartphones. Nowadays, Nintendo needs as many system sellers to keep their system alive. Every original Pokemon game is almost guaranteed to sell around 14m, but its hardware-moving potential won't be really any good on the 3DS, that is almost reaching its sales roof.

We never had a new pokemon gen releasing alongside a handheld. Could happen but never has. You said "Nintendo needs as many system sellers to keep their system alive", isn't that true for every console? And it doesn't mean the system sellers have to be there at launch.