Nobody, because that's not the point of the game

^ This.

They’re all pretty young for Arthur except Sadie and Sadie isn’t really an option since she lost her husband and dedicated her life to finding The killers.

Sadie without any doubts Mary Beth was like a little sister to Arthur much like Tilly and Karen while Susan was like a surrogate mother to Arthur. The only woman in the gang that Arthur had an affair in the past was Abigail but even so it was briefly

Charlotte Balfour.

I think if Arthur didn't die that's what would've happened