Ethyl Maltol is technically a sweetener; it doesn't sweeten per se but adds a thickness to the taste and helps blend flavors. I add it to everything, between 3-10 drops per 30ml: 3-4 drops for fruits, 5-8 for bakery/dessert, 8-10 for tobaccos. It doesn't take much, really. This is liquid EM which is diluted to 10% in PG. You can get pure EM crystals and dilute it yourself but that (I have found) is a PitA, and the diluted version isn't that expensive.
Acetyl Pyrazine is another 'base additive' that has a strong nutty essence to it. Also helps to add thickness and depth to flavors. I put this in everything except fruit, 3-4 drops in a 30ml. IMO it doesn't affect taste much at all in that quantity but will change the 'essence' of your vape.
Oak Wood and Hickory Smoke flavorings are super tricky, but great to have: I add one or the other to every tobacco and nutty juice I make. Oak is woody and very deep, hickory is strong and smokey. No more than 4 drops in a 30ml. The oak wood is a great alternative to tobacco absolute as a base for tobacco mixes; it gives the added depth without that grassy element that many people despise so much. Toasted almond and hickory, try it.
IMO juice will last about a year. I have a couple from last January: the flavor has deteriorated (2 are simply much weaker, 1 turned gross) but the nicotine is still fine. Not sure about raw flavorings.
With flavorings you can generally go by color - the darker it is, the more it is likely to gunk attys/cartos. With hickory that's fine bc you only need a few drops. Some flavorings, I believe, are best to just avoid. Cake Batter and Black Walnut, f.e., are suuuuper gunky, and require significant quantity to do anything, so to me they are useless.
