Some of the lost causes, you could consider a dilution. That gets you even more into DIY territory, but for example, cotton candy is just pretty much all sweetener. You could use that mixed with other mixes if you need more sweet, but adding more flavor to a cotton candy, that would be somewhat backwards, because it's over flavored, or disgusting to you at the strength that you've mixed.
I've "rescued" commercial flavors by using them added (in small quantity) to juices I make. Because I have DIY for a while, often commercial juices don't taste good to me unless I work with them. It can be as simple or complicated as you want to make it, but certainly adding some flavoring to mixes if needed is perfectly fine. Depending on what you add, they may or may not need a bit of steeping time.
I will say, down the DIY trail, my attempts to "fix" failed mixes taught me a whole lot more than my "successful" ones and i still had commercial eliquid as I went down the DIY trail, and you can be sure that many got amended, combined, diluted, added to, and sometimes binned.
Keep notes and let yourself be creative as you want to be! There's nothing to lose and a lot to gain. I LOVE DIY (I kind of had a hunch I would) and part of it for me is the endless creativity involved. I have successfully vaped every juice "win" regardless of what it may or may not have been, and it's just as fun (sometimes more fun) and just as educational (sometimes, more so) than starting from scratch.
Keep good notes, you will be building your flavoring skills, and good luck!
Anna