I'm for this. At the moment I think bipeds and post-hatchling dragons play largely separate games.

Part of the problem is our means of movement - and dragon riding would solve this. The other is multiclass and rating - but that's a different topic.

Re riding, I don't think we need mounted combat. With the way we eg. tail-whip, breathe fire, cast spells, biped passengers would be smashing their faces on our backs repeatedly during combat.

But the ability to carry them outside of combat would be good. It might require a special back scale?

Two extra ideas...

Rescue

In the heat of battle, a dragon can often escape by taking flight. Bye bye bipeds... I'd like to see an ability that lets us grab a (just one) biped and carry them off a short distance. It would have a timer on it which would force us to land after eg. 10 or 20 seconds. Then the biped could 'mount' properly if needed.

Dragon Rider biped school

If they were to introduce mounted combat of any form, I think it should require a new school, allowing only particular attacks that can be launched from dragon-back. They might also have special dragon-only heals/buffs.