This one was quite fun. The pumpkin added a nice quality to the soda, but it also added some unexpected difficulty. Prior to putting the beverage together I made chocolate syrup using Alton Brown's recipe. You can use commercial syrup (Hershey's and the like).I tried to do this as a yeast carbed product, and it was very tasty, but it had a lot of issues. I would recommend doing this as a pour over syrup style soda because I never got it to stop carbing and wound up with a very odd alcoholic beverage before it stopped.
So, here I will give you the amended recipe for the syrup.
1 cup roasted sugar pumpkin
2 cups sugar
4 cups filtered water
1/2 cup chocolate syrup
1 vanilla bean split
Essentially you will make a vanilla simple syrup and steep your pumpkin in it while it's still hot. Strain out the pumpkin pulp, and mix in your chocolate. Boom soda syrup. It's very very sweet so play with your syrup to soda water ratio til you like it.
Something I might try if I did it again is using either brown sugar or Steen's syrup in place of white sugar.