While I think Greywater Fastness is thematic and fun, I'm just afraid of all that can happen with an all shooting army in the Game of AoS
. Kharadron overlords was the first attempt at this, and though Cities probably does the better gun line army due to updated rules, I think you might feel more pain than fun when Khorne, Idoneth, Orruk Warclans get their first turn charges off at the beginning of the game.
That being said, you're far braver than I, Calam
! And I don't know the Greywater Fastness rules at all, I'm simply speaking about shooting armies in general.
At the end of the day, it's your armies, so don't let my opinion cloud what you think is fun and what you want to do.
I think it'd be cool to do balanced Hammerhal army of Greatswords, handgunners, and demi-gryphs screened by flagellant hordes... and some artillery like steam tanks. Just an idea