Make it from good quality wood, use plenty of nails and wood glue.
Brace it well against the tree with struts and even metal rio bar if you have it.
Drill holes in the tree and insert rio compliments of a hammer, use this for bracing..
As for the roof, depends where you live and the weather!?
Tin, sheet metal, or just plywood would do fine. Thatched if you got the skills!
A safe, secure and quality ladder of some sort.
Use some weather resistant paint or wood shield to make sure it stands against rot and insects.