For as long as people have understood that there will one day be three dragon riders, it's been widely accepted that they will be Daenerys, Jon, and Tyrion. Some argue that they're the three main characters, and that it just fits, while some more tinfoil theories suggest that – like Jon – Tyrion is a secret Targaryen.
I don't buy that. Not only would it be a massive cop-out if all of our favourite characters were secret Targaryens, but it would all just feel a little too convenient and tidy. If there's one thing George R.R. Martin's universe is not, it's tidy.
In the scene depicted above, Tyrion clearly had a connection with the dragons, but that means nothing. During the Season 6 finale Daenerys tells him it's not his sword she wants, but his counsel. He's not a man of action – he's a man of wisdom. Leave it to people like Jon to fight and fly – that's never been Tyrion's style.
If I had to guess, I like the idea of Bran being the final dragon rider – warging into Viserion, rather than actually riding on him. Being honest, I have no idea. But I can't wait to find out.