I don’t have strong feelings about allowing or disallowing postfix for, it’s just that items@for x {..} looks a bit strange and can not be read “naturally”, one reason for it is that in is not part of the expression anymore. On another side postfix for will work nicely with potential generator integration. Same can be said about if let. Either way list of allowed keywords can be extended at any time later.
As for sigil choice, I think we should choose from one-symbol sigils, so viable options are: @, #, ~. Exclamation mark probably should be excluded, as it will make a bit harder to parse code.