The problem is, the game is almost entirely built around the 5v5 meta. We have Twisted Treeline, but I'm not convinced that any particular guide or build I use in common place for any particular champ, would translate well in a mode that throws that meta out the window. Even in ARAM, some champions just don't translate well, because they either aren't built for early game, or are in on way capable of sustaining that type of mode with their kit.
Though I feel like arguing the length of any game mode is sort of difficult to argue. 1-2 hour matches I didn't mind, but it felt vexing after a mere couple to a few games back then. Yes, the trolls did feel less consistent, since the length of the matches probably discouraged players to throw the game. But despite how difficult it is to climb in ranked now days, it was a lot worse to try to do back then.
Edit - While I doubt this is possible to do anymore with how far the game has progressed, games built to last 40 minutes to an hour at most sound reasonable, even if it is just a thought to play around with.