Here's the thing: I notice that you're making mention of additional ingredients that are designed to help improve the flavor(s) you have. Something to think about is, sometimes a particular flavor cannot be saved, that, and, not all flavorings are created equal.
That's not to say that any given flavoring isn't
good, per say, rather, some just taste better than others.
With that being said, in my short tenure at making my own swill, I find that the key is experimenting with multiple companies until finding the 'one' so to speak. For example: If you're looking for a great strawberry, try TFA, LA, CAP, Inewera, etc. Basically, explore until you find what you're looking for, instead of manipulating the one flavoring you have (trying to make it what you originally thought it was going to taste like). Chances are, it won't, no matter what you add to it.
Now, as to the complex, multi-flavored liquids, that increases the difficulty a tad. Sometimes you can get lucky and mix a couple or more flavorings together that you think would go good together, and magically they do. I believe you need to taste each individual flavoring on its own to figure out what two (or more) flavorings will work together. This can be a hassle, and at the least, relatively time consuming.
But imagine, those vendors that have mastered the mix (or that make really decent
juice) have gone through trial and error, and you most likely will have to do the same. I am currently trying to master chocolate (God, help me), so my first order of business is to order mass amounts of various chocolates, vanillas, and any other component that is thought to bring out that flavor profile better, and try every single one at various %'s, until I either 1.) Figure out the potential of each said flavoring, or 2.) Quickly figure out that this flavoring isn't going to yield the results I desire.
As is, take your time (if you can), borrow some recipes (if you need to), try to hang on those who have similar taste profiles (I love some LorAnn Strawberry, if you loath it, we may not like the same things
). Bear in mind, you have an endless amount of knowledge and experience here to help you (in the form of all of us, yourself included), but in the long run, only you can prevent forest fires... er, I mean, create the perfect blend for you.
TL;DR - Buy lots of flavors.