The more I think, the easier it becomes to accept the possibility that it is Fluffy Heart that is the “shadowy alicorn”. All alicorns have a corrupt alter ego with one exception; Fluffy Heart who took over the throne after her mother at the end of S9, G4. The alicorns are immortal as long as this remains unanswered, and as the younger one, Fluffy will be able to survive into the G5. She was born as a princess of the Crystal Empire and will have great acceptance of the tribe in contrast to her mother and Sombra, as she is one of “them”. It will then make her very concerned about her realm and subjects, and it may lead to her voluntarily being corrupted to save the crystal ponies and others who accepted her.
Like the last alicorn - we do not know if Twilight is gone - it will be natural for her to claim the pony magic. The Crystal heart cannot generate magic by itself because it is a “failsafe” to secure the shield that protects the Crystal Empire from the polar weather and must receive magic voluntarily given with love from the tribe. We have seen that the mysterious alicorn has magic when she could look at Maretime bay without visible use of technology. Which may be based on the crystal magic. But she wants the pony magic because after all, there is a limit, and meant that it is her domain.
But we have seen that it is Sunny who has been given the right to declare the Unity Crystals as her domain, as she and her friends are guardians of the Unity Crystals. Her alicorn powers are due to the crystals that had chosen her, not the mysterious alicorn. That makes her a target for the mysterious alicorn. We do not know if there will be a long lasting hostile relationship or if Sunny will succeed in reforming Fluffy. But whatever the outcome; the mysterious alicorn must be converted to the “good side” during the G5.
“The Immortals” from the G4 - Celestia, Luna, Cadence, Twilight, Fluffy, Spike, Discord, Smolder and Ember. The IDW comics will show Discord which will play a minor role, while MYM and 3D series S1 will show Fluffy if it is her. Spike, Smolder and Ember may later appear, possibly in search of the dragon egg that may have arrived at Maretime bay. But it is possible that the fear of unstable pony magic keeps them away. It is possible that Celestia and Luna have ascended to the elsewhere and their magic is transferred to the celestial bodies. I do not think we will see them in the G5.
Then Twilight and Cadence remain, we do not know what role they will play. Twilight will meet Sunny, this seemed inevitable, especially if there is a bigger threat than the mysterious alicorn.
The movie has left several hints about what may come, we can now confirm the hypothesis about the Memory Stone as collective memory loss has been revealed in MYM.
1); a drawing of the equestrian flag indicates a connection between the royal sisters and the celestial bodies which in G5 apparently moves themselves.
2); A drawing of Twilight Sparkle suggests an upcoming role in G5.
3); Footprints of a dragon indicate that the dragons still exist, confirmed by the dragon egg in MYM.
4); The Mane 6 and the elements are shown, it may seem a source of inspiration, the Mane 5 will take their place as the heroes of the ponies.
5); Zebra mask possibly means that the zebras and non-ponies are out there.
6); The portrait of Starswirl may be a hint about Argyle and his role, that there are secrets that the Mane 5 must find out.
7); A drawing of the harmony tree is confirmed by the movie, that it exists.
8); Neighagara and Manehattan may still be in existence, we have not seen the northeastern parts of the equestrian east coast.
9); Grogar’s bell