Champion Interactions Beyond the Taunt Button
So Riot's got a bunch of character interactions sprinkled through the VO of champions, but there's a little problem: they're locked behind the manual activation of taunting (or in rare cases joking) in close proximity to another champion.
These interactions are good to have, but because they're effectively hidden away, most folks won't ever experience them and only know they exist through videos or the League wiki. It's a damn shame really, but there is a solution, one that Riot's been slowly moving towards:
Interactions that trigger automatically and aren't tied to the taunt function.
We've seen this recently with Kled and Camille, where they diss folks they kill. Really mean, but really great for character interaction! Having more interactions like this would be great to see, and there are ways of doing this beyond just dissing someone's corpse. Overwatch is great for this, with such interactions being tied to the deaths of allies, in spawn conversations and ability - ability interactions. One such example is one between two characters: Ana and Pharah, where if Ana uses her sleep dart to interrupt Pharah's ult, Ana has a specific line of dialogue she'll play upon doing it. It's really cool and I'd love to see more things like this in League.
The kill interactions are good, but we can do more! So I figure I'll just paint a picture using a few examples I conjured up, though keep in mind these are just examples and not exemplary literary works:
kills an enemy champion while an allied
or
is nearby:
"Watch and learn girl, this is how you use magic."
sees the death of an allied 
"Master no!"
dying to an enemy 
"So this is how it ends...brother."
being healed at low health by 
"You've saved yet again Soraka!"
TL;DR more champion interactions beyond just having special taunts would be great Riot. Kill interactions are great, but there could be more.
kills
"Oh jeez....Sorry, Cupcake! You'll walk it off!"
sees an allied
die
"Weaklings...Ha!"
uses ult on
"IF YOU SCREW THIS UP ILL PERSONALLY FUCK YOU UP MYSELF, BEAR
sees
"You're adopted!"
see's 
sees
"Tryndamere, my greatest creation."
spotting Enemy
:
When a nearby Ezreal gets a kill:
"How dazzling!"
could mimic the music made/used by other champions, such as Kled's ult or Tahm's tongue-pluck. She could even play the music from other maps or the lobby.
sees
"How long have we been stalking this one, wolf?"
"Too long! The chase ends now!
sees
"Are you my nightma- Ugh!"
Charms
"Machines and fancy tools are no substitute for proper skill"
sees
sees
ally or foe