How does DF get traders to disembark from their wagons? One idea was for the trader entity to be "bundled" with the wagon entity as just one more good, that then is removed from the wagon at the Depot which causes the "Merchants are setting up their goods" thing. What's interesting, if this supposition is true, is that entities can become goods which will be able to be stored and moved, (by wagons or adventurers or creatures with backpacks renamed as "saddles", ect...), and that the entity is able to be unpacked into a self-motivated object once more preserving its statuses.
I don't know if that's how it works, but the fact that enemies can come in on mounts, and babies are mounted on their mothers tells me that there's code for that already written, and that finding out how that code interacts with the surrounding matrices might indicate how adventurers or fort-dwarves might be enabled with that same functionality.
The mine carts can be ridden in - I wonder if mine carts could be changed to automagically lay their own tracks? Seems to me, that a mine cart able to do that sounds like an adventure-mode version of a wagon. Add a creature into the mix as a motivator (or a team of creatures if you like) and the mine cart/adventurer wagon is self-powered as well.
I also wonder if wheelbarrows are able to transport people as well as rocks? Palanquins and jitney carriages sound functionally similar.