Thanks for the ideas on adding nuts.

(nuts,, almonds, same type of ingredient). I always do like experimenting. I have ran across numerous recipes for streusel, and some include egg, and some use powdered sugar instead of granulated. Have not tried these yet.
Wonder how almond flour would work?? I may get a quantity and use it in my experimenting recipes of all type.
@Sally PB ,, does almond flour flavor cakes with almond flavor??? You are the first person that came to mind asking about GF flours.

As to vanilla added to streusel, I usually add an envelope of this,, but ran out.
Just to calm fears, and set your mind in a GREAT MOOD.

You will notice that this recipe ,,,,, while
not CALORIE FEE is low on butter (fat calories) ingredient as a total. (compared to other recipes I have. ) Many recipes also suggest oil in them. I have modified such to replace oil with apple sauce, and such. Stuff still tastes great, but with the lower amount of fat,,(butter or oil) the cakes do not stay moist as long in the following days.. That translates to this..
MUNCH IT UP 

THE FIRST DAY MADE,,, AND ESPECIALLY WHEN STILL
WARM.