I've found there are three very effective ways to lose money as an arborist, 1) Agree to build a treehouse, 2) Agree to hang a swing, 3 Agree to clean out gutters unless you can walk the roof and zap em with the power blower.
I'm not trying to discourage you at all. Everyone finds their niche. It's just that you don't need arborist-type experience to build a treehouse, you need experience in the construction trades. As far as the tree goes, use common sense; don't rabbet into the trunk, know the tree is going to grow, know the higher you build the more the tree will move and know that you may possibly be connected to the lawsuit when the kid falls out of the tree and kills himself. Kids just get hurt. They do stupid things (...looking back on my own childhood), they... they're kids.
If you do move forward on it, build low, build on independent posts to support the treehouse and avoid building on or in the tree itself. The higher you go, the more of a logistical hassle it is, unless you have a bucket truck. If you have a bucket truck, you really should be out doing tree care. If you have to rent a manlift for the job you better have some really wealthy clients. If you're up and down and up and down on ladders, life sucks quickly. Kids just want a fort up off the ground and a place to go make out and smoke pot when they're teenagers. My advice, keep it low, keep it simple because YOU are not going to enjoy the use of it unless it's on your property, or you build for a relative.
I'm coming from a place where I agreed to do a relatively simple treehouse a couple years ago. A one-week job spilled into three and there was a major storm that hit our area during week two (when it should have already been finished). Treehouse client had a party coming up soon for their kids and it put me in a spot of a deadline, which I don't do well with, especially whan I have to shelf my bread and butter business.
There's something romantic about building a treehouse, and the end result can be very cool. Building in trees is much more difficult than building on the ground, though not impossible.
Here's a few
treehouse links. Also, the book
Treehouses of the World, wow, what a knockout book. Especially note Sting's treehouse. Treehouseworkshop.com is where you need to investigate.
Good luck.