Well sometimes you don't need any additives at all, as some tobaccos taste good at their own at the right % and with a good PG/VG ratio. For Hangsen, I'd recommend starting with 3% at 50/50 PG/VG. Steep for a week, sample them, and adjust from there.
Some common additives to smooth things over include EM (Ethyl Maltol 10% Solution found at TFA), ACV (apple cider vinegar, although some just use plain vinegar), TFA Smooth, MTS Vape Wizard by Flavour Art, TFA Sweetener, and various types of creams (bavarian cream, sweet cream, whipped cream, etc).
Some common flavorings that people use with tobaccos to help them taste more sweet/complex include peanut, almond, vanilla, caramel, butterscotch, chocolate, coffee, hazelnut, pecan, cinnamon roll/danish, graham cracker, cake, honey, molasses, and brown sugar.
Some common fruits people add to tobaccos include cherry, peach, apple, blueberry, watermelon, and coconut.
Edit - You should start by sampling the tobaccos by themselves to figure out what sort of "base" you're working with. Is it a dry tobacco? Either make it drier and more 'western' like by adding almonds, peanuts, etc., or make offset that dryness with something moist, like small amounts of cake batter, vanilla, creams, etc. Is it a sweet base? The possibilities go on and on.
I'd start by just getting an idea in your head of what you generally want. Pipe tobacco taste, cigar taste, non-descript tobacco taste, dessert tobacco, fruity tobacco taste, menthol tobacco, island tobacco, etc.
Be warned though, a realistic cigarette is near impossible because A) the damn things on fire, and B) there are over 5,000 chemicals in there that we can't put into our ejuice. Yes you can get 'close' by using certain bases and certain additives, but imo, it'll never taste exactly like a leaf on fire wrapped in fire safety chemicals and doused in factory additives. I think what people hope to aim for the most is the scent of certain tobaccos not lit on fire. If you can get a vape that tastes just like the smell of fresh, unlit tobacco, than you'll most likely enjoy it.