Casey,
Nice boat, so you know how to work with wood.
Thinking out loud with you. I tend to think outside the box most of the time. If you really want to build the frame out of wood you can do it, although, like Joanne, I'm not quite sure why you would want to go to all of the trouble vs. going with a steel frame.
You'll have to basically do your own designing on the frame. As Andrew mentioned, avoid the half laps if at all possible.
Pros....Aircraft wing spars can be built from wood very successfully. Wood essentially doesn't fatigue and you can call it Nature's composite if you want to make it sound cutting edge. The coefficient of expansion will be just the same as the plywood of the floor and any wood used in the sides. You know how to keep the water out of the equation, at least for a while, so rot shouldn't be a problem in the short run. As I recall Douglas Fir is relatively light, but has a tendency to split more than some other woods, so you might have to watch how you fasten to it. All in the design. Have you noticed how many steel floor joists we have in our homes?
Cons... You may have to fiddle around with making custom steel hangers for the springs to effectively distribute the loads to the wood. Same thing for the ball coupler. You have to add in the weight of the hardware and fasteners. I think that it wouldn't have to weigh any more, maybe less than steel if you were to push for some serious design. The problem is that there is probably very little empirical data on wooden trailer frames in the 21st century. The steel framed trailers work quite well and they would be a whole lot less work if you count the time trying to figure things out. If you overload a steel framed trailer, things tend to bend a good bit before they fail by breaking. You won't get that in a wood frame, so you have to overbuild a bit more.
I think that's why most folks on the forum will still point you toward a steel frame. It works, and if it ain't broke, why fix it?
J.B.