I agree with rolling with PEA-Martinelli-Balogon/Nketiah at Striker. Also agree that Martinelli can develop holdup play skill, and maybe do a little more of a false 9 setup, kind of like Firmino, replacing Lacas current role.
As far as CBs, I think going with Gabbi, Mari as your left footed options, and then for the future, you have Saliba and Mavropanos on the right side, you have a good setup, you just need someone to provide senior leadership. Maybe that's Holding? I still think Luiz could do it one more year although I know no one wants to hear that. I have heard rumors that Arteta will indeed offer him one more year. I think RB and midfield (left footed 6) is where we really need to invest this summer.
I've always been a huge Martinelli homer (at least since his first appearance in the Europa League last year) and I think the way that Arteta likes to progress on the wings and send crosses in is particularly suited for Martinelli. He is great in the air, and that video just shows how well he can shoot off of crosses.