For the rhubarb simple syrup: Add the rhubarb and 2 cups water to a small pot and bring to a boil. Reduce to a simmer and add the sugar. Stir until dissolved, then take off the heat. Let steep until cooled, about 2 hours. Stain into a resealable container and refrigerate until chilled.
For the strawberry-basil puree: Combine the strawberries, basil and 1/4 cup of the chilled rhubarb simple syrup in a blender and puree until smooth. Transfer to a pitcher.
For the lemonade: Add the lemon juice and 1 1/4 cups of the chilled rhubarb simple syrup to the pitcher with the strawberry-basil puree and give it a good stir. Add the seltzer and stir lightly. Pour into 4 glasses filled with ice, garnish each with a lemon wedge and basil leaf and serve.
Store the leftover rhubarb simple syrup in a resealable container in the refrigerator for up to 2 weeks.