Akainu's got his work cut out for him and I can't wait till we see him back in action.
Everything relevant about Shanks and crew has already been said I guess. As far as his smile in that arm-losing panel goes, I don't see it as being evil at all.
In that moment he was just prepared for what he was about to do - bet his arm on Luffy and his future. He was resigned on losing it and the smile was more sad than evil.
Also, him declaring it's time to go after the One Piece will be a point of contention for sure. What did he mean? Personally, I think he meant the times and circumstances were such that goimg after the One Piece rn is of paramount importance.
He did not necessarily mean that the Red Hair Pirates are going after it - the same declaration can also be directed towards Luffy and his crew. Shanks now knows Luffy is at the summit of the pirate age and it is expected that Shanks might give him a hand (heh) to make that final step.
Ryokugyu won't be reduced only as a hype tool for Momo but the "Yamato stay out of this" line from him obviously implies that we're due for another important moment from Momo probably regarding his physical strength this time