Aren't we going to be running out of keys eventually? Would a meta key like used in Emacs (please, no editor wars 
) work? Like [Esc] and then at the prompt it would display META or something, at which point you can press R for reply, F to flag, I to eat ice cream, and so on.
I'm really fine with any key just as long as Reply can actually be done from a keyboard shortcut. Many "regular users" (
) have major difficulties understanding that this is not a 1:1 conversation like their SMS or WhatsApp app