A word of advice, before you ruin a ton of
juice, go easy on the Sweetener and Ethyl Malitol, it can and will ruin the flavor by masking it or making it taste like something more or less than it really should. I would also recommend one or the other in your
juice, not both. EM acts as a sweetener on the receptors to make your taste buds sense sweetness when there really isn't any there. The sweetener is going to deliver sweetness and it will physically be there (some sweeteners can also shorten the lifespan of coils / wicks as sugars burn, cake on, and keep on until you get a nasty burnt up harsh flavor).
Mixed, Ethyl Malitol has a spun sugar (i.e. cotton candy) like aroma, which sort of gets passed into the flavor of your juice if you overdo it. Stick with 1-2 drops (of either) per 3-5ml of DIY Juice. If that's not enough, go to 3. It's time consuming, but it's better to step it up drop by drop than to throw away product, regardless of your costs.
As far as other additives (other than PG/VG and Nicotine if you want it), none would be my recommendation. Menthol or Creme De Menthe will cover our menthol additives (and go easy on them too - they are *strong* - 1-3%, 4-5% is pretty much overkill). Juice should remain simple unless made complex by flavor modification, not by additives. Of course, each persons' flavor requirements are subjective to their own personal tastes, so mix a little and move up until you find what works best for you.
There's no right or wrong way to really mix - some you'll love, some you'll hate, and some might just become the glorious All Day Vape

.