For me it is simple, form follows function. When designing a thing the function needs to be consider first. I believe that if something has been designed to function properly, the rough aesthetic falls in to place and then a human element, style, or refinement of design can ride on the coat tails of function.
When designing a timber frame structure, we just need to consider what our goals are for the space and apply some math and engineering. For me there are always two goals when it comes to designing timber framing and that is one, the space must be warm, comfortable, welcoming. Creating a cozy and comfortable space is easy, it is just automatic with a timber frame.
Second, try to design all the joinery to work together. Designing joinery is a thing all by itself. The goal is use joinery and geometry that comes together in such a fashion that no fasteners are needed. This allows the timber frame to behave as a single structure, transferring loads from purlins, to rafters, to wall plates, and then down the bearing points to the foundation of the building while being support by all the braces. No fasteners, just geometry and some thought.
There is a porch roof on a restaurant near my house. The carpenter tried to make it look like it had been timber framed by using reclaimed timbers and used fasteners to hold the butt joints together. If the design was executed properly, he could have pulled off the look. Every joint on this porch roof is in tension, so without the fasteners, the structure would fall into a pile of timbers. As a result, it looks uncomfortable and uneasy. Form follows function.