Oki, I was thinking something like this-
Non flying characters can be picked up by flying characters if they are half or less than half of the size of the flier. And to be picked up, they have to be in the same party or group, and/or friends. I'd say there could also be an option where they could block a certain person from picking them up, even if they requirements are met.
I dunno if this could happen, but it would sure be cool!