I have found a successful caffeine replacement for my Coke Zero: V8 VFusion + Energy! Honestly, I thought it would taste like crap, but it was the only caffeinated drink that wasn't a soda or an energy drink loaded with way too much energy/caffeine. Much to my surprise it tastes pretty good, even for awful sounding combinations like pomegranate blueberry. As far as caffeine, it has more than most diet sodas, but still less than an average cup of coffee, so I can have it with dinner (as I did with Coke Zero) and not be up all night.
It does have 10g of sugar per can, but it also has 1/4 cup of vegetables (which I could really use more of) and 1/4 cup of fruit, quite a bit in the way of vitamins and such and only 50 calories. So, we'll see how it does as far as weight loss as a Coke Zero replacement.