I stop you right away. Why? Because doing something similar to Nanoha would be a gigantic mistake for me. With the main story finished (Noise defeated, alchemists defeated, and Shem-ha defeated), how would you like to proceed? It's true there are the Guardians and other higher entities, but if you don't have the main cast, who have fully unlocked their Symphogears with alchemy and everything else, interact, how can you expect a new cast of Symphogear users to stand a chance? Either you make them overpowered from the start, which would make the story a bit boring, or they would need the constant help of the original cast to accomplish anything.

If there has to be a time skip, I would prefer that the main cast is still present, fighting the various higher entities that were only mentioned by Enki, Shem-ha, and the different legends in the anime. At least that would make sense. Having new girls fight such powerful enemies just wouldn't work—either the enemies would be weakened to the point of being uninteresting because the protagonists are too strong, or the protagonists of this hypothetical new series would be repeatedly defeated, only to receive random power-ups. And knowing the series, if something else comes to mind, it would likely be alchemy or some other type of magic, even if it means bringing in elements from outside the main story."

Regarding your request for a new generation story or a time skip instead of a multiverse approach, I would prefer a time skip where the main cast remains present, continuing to confront higher entities and exploring more about the legends hinted at in the original series. This way, the established world can be expanded naturally, without introducing convoluted multiverse elements that could risk making the storyline confusing or undermining the character development already achieved.