From a structural standpoint you could not go out very far unsupported. Even notching out the stump, 2x6s wouldn't go too far, maybe 4 ft or so without braces of some kind. If you would want to go out farther than that without knee braces or steel I would suggest larger girder lumber such as 2x10s or 12s and half lap joinery into a tic-tac-toe shape. The stump would just be cut into a square (24" square?) in the top 11-1/2" ans 2x12s around it. You would just have to make sure you reinforce the cross joints of your laps with deck bracing, being that you basically just cut their strength in half. Still without knee braces or steel gussets of some kind, I would be worried about the stability past 6 ft or so. Depends on the amount of weight and grain strength of the stump. A person standing 6 ft away from the central point puts a lot of torque on that stump. Dry dead stump = no stability. All depends on how much you want to put op there.