some like to use capsaicin, yes. there are vendors selling juice with it already added (i think this would be best) and some people also add grain alcohol to their e-liquid (tiny amounts) to increase the throat hit. adding water would LESSEN the throat hit, i would think. I am unsure of whether or not exposing a juice to oxygen would make the nicotine harsher, with better throat hit... but ive vaped e liquid thats expired for over 2-3 years, and, it never LOST it's TH.. I am unsure, however, it the throat hit had ever gotten better. I DID have a bottle of blue raspberry e liquid that i could not vape when i got it, because the throat hit was not strong enough for my liking, it was spongy and unsatisfying. the sour additive also made me cough.
I just finished that bottle 2 days ago, after having it in my vaping chest of draws for over 1.5 years, and the TH had gotten better for sure... It was unvapable when I received it, and when i vaped it last week/this week, it was very pleasing. the sour had reduced and the TH was a fair bit heavier/less spongy. So, if this supports your belief that oxygen improves the TH, there ya go.

i've vaped MANY an expired juice and they never seem overly harsh, however.
vaping 100 percent PG is very satisfying as far as TH goes. I find that VG really thickens up the smoke, making it feel cooler on the throat, milkier, and less harsh. VG also makes my chest feel heavy after 2-3 days of vaping 100 percent VG juice. Not cool. I have found that nicotine content is the best way to control the throat hit, which is why i sometimes vape up to 24mg, even in my sub-ohm builds... TH is my MOST IMPORTANT saving grace when it comes to being able to vape, rather than smoke. I was a chain smoker, so, i never felt the light-headedness or high that slow smokers feel... likewise, i am a chain vaper, and, the MAIN thing that i am addicted to, is the throat hit. I could honestly give or take flavor and vapor production. TH is very important...
I have heard a ton of people recommend grain alcohol. capsacin is supposedly another TH enhancer, altho ive heard that it enhanced the TH in ways that are unlike the TH that myself and probably yourself, are looking for... it makes the juice HIT and BURN your throat, from what i've heard. not what im into. I recommend you try 100 percent PG and maybe some grain alcohol, at 12, 18, even 24mg.