Shrinkage can be a challenge, we stack post on post with timbers attached to sides of posts to reduce vertical shrinkage. Also, if the posts are shorter than the beams you can create sockets to drop the post on the next level into. Non-traditionalists can save the client a lot of money by implementing steel strapping, or internal steel fins bolted and counter sunk, holes filled with short pegs. One very important consideration is that each level be stable in and of its own, without relying on the others. Also, it is very helpful if the there is a complete floor system at each level that can function as a horizontal diaphragm to increase stiffness. It is very important that bracing be truly structural and not decorative, in that it be housed and not simply a tenon, and remember that braces do their work only in compression. For the truly challenging frames there are external solutions that can be applied to the frame under the panels or even within the panels. But as has been pointed out, a qualified, experienced timber frame structural engineer is absolutely essential.