Another note:
Sunny says in what is essentially a throwaway line that she lost most of her Dad’s belongings in the Lighthouse collapse D:
Sprout is worst pony
Then where is Grogar’s bell? If I remember correctly, it is indestructible. It should have been found and kept in the cellar at Brighthouse, but we have seen no sign of it. Could it have fallen into the sea?
Also, it’s possible that the strongest magic item in G1 might have appeared, because a Breezie gave a locket to Sunny with a mysterious phrase; “It has long been waiting for a special pony. A hopeful pony. I believe that You are the One.”
We know what the locket is capable of, it contains deadly magic in the form of the Rainbow of Light and is perhaps the most dangerous thing ever created by the ponies in all Generations. If it’s the Rainbow Locket, this is overkill against Opaline, even if she reaches full strength. It was this that destroyed Sombra when Twilight channeled the magic from all six ponies into a magical beam very similar to the rainbow of Light, according to IDW comics he was pulverized into dust.
Megan Williams in G1 had come to the aid of the ponies in their darkest hour, and she had helped them, mediated between them, been like a mother to them - and she had been their hope, that if she was a pony, she would completely certainly have the Element of Hope. She let the ponies do as they pleased, only interceding for them and helping or advising them like a true leader who cares about them.
Sunny in many ways can end up being like her, because she doesn’t want power, wants to help others, treat ponies equally, and brokered between them like when she got her cutie mark. She holds onto her ideals so that she is more or less incorruptible in the face of Opaline, who believes the young alicorn is the biggest threat to her plans.
It’s very possible that the Rainbow Locket couldn’t be used after Megan’s time (if she was in G4’s past - but we’ll probably never learn) because it predicts a pony with the right traits and the right element - like Sunny’s personality and element. And Sunny is the element of Hope, which had reunited pony tribes. In C5 we have seen that she is back where she was in the movie with leadership skills, especially in connection with Zipp around Misty. She is back in the leadership position of the New Mane 6.
Maybe the locket will be essential to stop Opaline in C6, maybe to find Twilight - but the resemblance to the Rainbow Locket doesn’t seem to be coincidental, and it can’t be the Fire Ruby Spike made for Rarity. Then a question arises; if it’s the legendary locket - which Sunny can only use - it can’t just be meant for Opaline, has Berrow decided that Opaline should be killed? Or that there is a much bigger threat that had to be prevented?
We don’t know what Opaline is going to “open”, and everything indicates that it is on this basis that the finale is going to happen. There are three episodes in C6, and from what we’ve seen of C5, it could mean that the special SoS has been added - for ep. 5 and ep. 6 may have been original C6 episodes. Then it means that ep. 1, 2 and 3 in C5 will be about the dragons, cutie mark theft and the final confrontation between Sunny and Opaline.
SoS could be the finale of MYM where the secrets are going to be revealed and it is possible that it could be where Twilight and Sunny will meet each other. Then the last act in the movie about the transition from G4 to G5 will take place.