We have an at-home carbonator (soda stream) because I got hooked on club soda with fresh lemon squeezed into it when I worked as a bartender. This is mostly the type of carbonated beverage I drink - club soda with lime, lemon, a bit of unsweetened juice, etc. (I make a UTI-buster that's unsweetened cranberry + apple cider vinegar topped off with club soda - it tastes a little crazy the first time, but it sure works!)
Personally, I'd go cold turkey off of the dew. Whether it's diet (aspartame) or regular (HFCS), I believe it's got all kinds of fun chemicals, like brominated vegetable oil (EW... just ew. now go google it.)
If you want to wean yourself off the dew, here's some ideas with club soda - if you're buying it instead of making it, make sure the only ingredient is carbonated water (no salts, sweeteners, etc.):
Start watering down your dew with club soda, gradually increasing the amount of club soda... as your tastes change, straight dew will start to taste gross (WAY too intense) to you.
I've used the sodastream syrups before (especially for guests who can't fathom soda water or unsweetened juice + soda water), but they either have Splenda or splenda + sugar, so I almost never use them anymore. They do have a mountain-dew-esque flavor which might help you to wean yourself off the dew (just add less and less of it each time); while it has fewer chemicals and bad things, it still has them.
I'd actually recommend fresh-squeezed grapefruit juice mixed 1/2 & 1/2 w/plain fizzy water... this concoction will never taste the same as mountain dew, but we enjoy it every now and then (sometimes a splash of unsweetened cranberry too).
I suppose if it's not sweet enough, you could add some stevia and wean off of that.
hope that helps.