Perhaps several levels too many. Would make it even *harder* for newbies to get into the game (whether or not this is a good thing, debatable.)
If what terrain types you're crossing matter, it'd make embarking on terrifying terrain fail before you even get there. You'd lose too much to the inevitable waves of ambushes by zombie whales or whatnot. Probably similar for glacier or deep desert forts. You'd be limited to aluvial, or mountainous regions, that are relatively close to the mountainhomes. Might have to play through several forts (and presumably at this point have a way of deeming them 'successful' and abandon for new fort without leaving them to animals) in order to set up logistics to get anywhere *interesting*
It'd also make it harder to set up minimalist-startup forts. You'd probably end up killing yourself trying to get the fort to begin with the one anvil and one copper nugget that you want it to, and none of the extra food, wagon axles, medical supplies, etc that you'd need to pack to have the dorfs survive the trip.
Possible solutions:
Make it turn-offable by init (probably recommended regardless)
Have the embark screen tell you exactly how many resources the dorfs will use in their journey (with chances of losing random dorfs/supplies or animals being slaughtered for food increasing as you deprive them)
Make it a whole other mode of the game. Let's Play Dwarf Caravan! Ply your way across the World! Trade at several mountainhomes! Ford the river, caulk the wagon, or make a bridge across by pumping magma up and damming it! Avoid the dangers of Wagon Adaption! Piss off the elves before you ever reach sweet OregonAles The Bloody Hall of Sorrow, your new home in Fortress Mode! (switching to fort mode being of course an option, or just send out more caravans in order to shape the Great Dwarf Empire)