Combine warm water, 1/4 teaspoon sugar and yeast in a small bowl. Let sit 5 minutes until yeast is foamy.
Combine 1/4 cup sugar, salt, 1/4 cup butter and vanilla in large mixer bowl. Add warm milk and stir to mix. Add yeast mixture, egg and flour. Beat on medium speed until well blended.
Spread into greased 13 x 9-inch pan. Brush with 2 tablespoons melted butter. Cover and let rise in draft-free area until doubled, about 1 hour.
For the Strawberry Rhubarb Filling: Combine rhubarb and water in large saucepan and cook over medium-high heat until the rhubarb is soft and beginning to lose its shape.
Add in strawberries and cook for an additional 2 minutes.
Stir together corn starch, sugar, and gelatin powder; add to rhubarb mixture.
Continue to cook and stir until the mixture is thick, 2 to 3 minutes. Remove from heat and use a potato masher or fork to mash the fruit pieces. Allow the sauce to cool before readying the cake for baking.
For the Streusel Topping: Combine all ingredients in a bowl with a pastry blender until butter mixture is the size of peas.
Before baking the cake, gently make small wells in the risen coffee cake, about 1 inch apart. Spoon about 1 teaspoon of strawberry rhubarb sauce into each well.
Spread the remainder of the rhubarb filling evenly over the top of dough. Top with streusel topping.
Bake in a preheated 350°F oven for 30 to 35 minutes until the top begins to brown.
Cool on wire rack for at least 15 minutes.
Combine all glaze ingredients together and drizzle over coffee cake. Best served warm.