#3 The Shield names a massive replacement as the third member
There's literally nothing for Braun Strowman to do in the roster right now. He's been waging a war against Baron Corbin but even that seems to have run its course because The Shield is facing the heel trio next.
Let's assume for a moment that Ambrose is not too keen on joining the Shield or indeed, turns on the Shield as mentioned in the page before this. The Shield will be a member short and they could choose a monster to join their ranks.
Having Braun Strowman as a member of The Shield would actually work in his favour because it would make him a serious character and not have him be booked in the goofy segments that he always seems to be relegated to. It would be a genuine surprise and get people talking.
And there's nothing else Strowman can do as a babyface right now.