Playing with randos, 1v9 and 2v8 both have serious problems. In a given 2v8, a couple of people will immediately leave because they wanted to be Impostor, and then the Impostors tend to win before anything interesting can happen. In 1v9 you can play for hours and never get impostor, and it's fairly trivial to find one impostor.
Suppose a match starts with 7 Crewmates and 3 "Impostor Candidates". The Candidates know the identity of the other Candidates, and become a full Impostor once the other Candidates are dead. The Candidates can kill each other, but cannot kill Crewmates until the other Candidates are dead.
This triples the chance of getting to play as a killer compared to the 1v9 scenario, while the Crewmates ultimately only have to deal with one Impostor.
Candidates being able to kill each other could potentially be weird because it comes down to who presses the button first (and server latency), so the strategy will be to stay near Crewmates so that you can't be killed safely, while looking for opportunities to pick off the other Candidates without getting caught. In a Candidate vs Candidate fight, the winner is whoever is faster to react to the opportunity of sneaking a kill without getting caught. In other words, you need to be slightly greedier than your opponent, without being so greedy that you get caught.
A consequence of this setup is that Crewmates cannot be killed for the first few minutes. This gives them a chance to gather intel and try to prolong the fight between the Candidates, and in exchange they have to deal with the inherent chaos of 3 killers running around.
Naturally, meetings will still happen in the earlygame, giving a way to eliminate rivals without directly killing them.