Charizard'. Of course it's promoting the new games, and it was a very clever way to go about it. I for one never expected anything new out of it, I was excited for the series for nostalgia reasons like many others.
The megas were revealed before this, it wouldn't have been much of a surprise if it was a mega we already knew about. The series was a perfect way to introduce a new mega, and personally I LOVED the way they introduced Mega Charizard X in the battle against Mewtwo. Red was pretty clear in his reasoning for why he chose Charmander; it went with his name. Charizard was and still is a pretty popular Pokemon (it was the first Pokemon I ever trained, too.) If it had been a secondary Mega Blastoise or Venusaur, then he probably would have chose that, but GameFreak had went with a secondary Charizard Mega (probably most likely due to its popularity), had Red choose Charmander for his own reasoning to lead to its introduction. They certainly didn't have to include any Megas, but with the games coming out shortly after the series did, they certainly didn't waste their opportunity to promote them and give everyone another reason to be excited for the new games just as well.
It's also true that any other trainer could have had one- but it goes back to that Red chose Charmander, and the Mega Stones were seemingly as unfamiliar to everyone in the series as they were to us at the time. If there wasn't a second Mega Charizard, Red (or whatever other trainer they chose to give the Mega Stones to) could have picked any starter Pokemon and they would have went with promoting that one, but there wouldn't have been anything new or exciting about it, not nearly as much as a brand new mega. They wanted to emphasize how unknown and special the Mega Stones were, so of course the main character would be the one to receive it and use it for the first time. That's what made the whole thing so ~special~
Watching this live and being like "what the fuck is this new Charizard oh my god" is by far my favorite Pokemon anime experience, nothing ever caught me off guard as much as that did. I loved the series as a whole even before the reveal and could already stand on its own for what it was, but that reveal definitely made it more special.