Guaiacol is great to add smokiness to the flavor. You are right, too much of anything will ruin a mix IMO. Too much vinegar cancels the flavoring altogether. The FE Vanilla is one that I will say is a must have because it can be mixed with any other flavor and be good. My top FE flavors are Mango, Blackberry, Apricot, and Vanilla.
I like fruit and sweetish flavors so I always have FA Cookie, FW Sugar Cookie (not as sweet as it sounds) TFA White Chocolate, TFA Bavarian Cream, FA Marshmallow, FA Whipped Cream on hand at all times in order to sweeten, add depth and smooth out my fruit/sweet type flavors.
Not sure if any of this is useful to you since I am not a tobacco fan