A few things I've heard of ...
If it's not truly gross, mix some of the more likely combinations and try them.
If it's really bland, cover up the flavor with DIY flavors that are compatible. I have some in this category and will probably just swamp them with something like menthol, cinnamon, chocolate or citrus.
Get a big bottle of premixed unflavored ejuice and dilute it down to the point where it's bland and then flavor it.
If it was high end, someone probably likes it so either PIF it or sell it.
If it was cheap, strong and awful, toss it. I.e. if you wouldn't subject the homeless guy on the street to it.
