I realy like this idea and have thought about it before, if never posted. I agree that new expeditions should be able to be sent from existing fortresses, but I personaly dont think they should have the 7 dwarf limit or limit on how much they can take.
If you have gone to the time and effort to make a huge fortress and gather enourmus wealth, why should the next expedition be limited to 7 dwarves and highly limited supplies. You should be able to assemble a fleet of wagons filled with lots and lots of supplies and send practicaly as many dwarves as you want out with the expedition, and plenty of animals and masterwork goods too.
Making wagons should be simple, costing 3 logs, or more if wanted, and should be constructed outside with a clear path to the map edge. It should require an architect and carpenter. Once made, it should have a large capacity for goods. To embark, they should need 2 draft animals and at least one dwarf to guide each wagon.
For settling a new fortress, other animals could be assigned to follow/be tied to the wagon, and the other dwarves could walk. The wagon and settlers should have to walk/roll to the map edge, then the calender should advance however long is reasonable for the wagon to get to the destination. This time should be dependent on how far the new location is from the old location. For the forseable future at least, the wagon should essentialy teleport to the new location, ignoring blocking features in the way for simplicity.
This new wagon system should allow for trade caravans to be sent out too, but with slightly different mechanics. It should also be able to perserve your original wagon for future use, and move it around following the rules of wagon movement with two pack animals.