Now that you mentioned it, The Bewitching Bell is going to cause a lot a problems since magic has come back.
Yes, there are indications that Grogar will appear in the G5, most likely as the “End boss” that Sunny may defeat to complete her desire to return Equestria (and the world) back to friendship and harmony. We do not know for sure. But it is possible that Grogar may have been thrown into the limbo many millennia ago by Gusty the Great which made his return impossible. The case of the ponies living in fear of each other, the disappearance of the Everfree and the suspicion of memory erasure along with the loss of active magic points to an outsider and powerful invader. Which may not yet be strong enough to break out of the limbo.
But Twilight had made sure that the bell had been emptied of magic, as it was little more than an instrument that could take and store magic in infinite amounts. Only the Terrible Trio who may have destroyed the book that explained to them about this, knew how to use the bell, but as long as it was not empty. It is thus very unlikely that anyone other than Grogar himself can use the empty bell. As long as Grogar is prevented from breaking out, the bell is harmless.
Idea; what if the Sombra in the G5 is not the Sombra from the G4? The last time we saw him was when he was a smoke monster like an umbrum as he was escorted away from the transformers’ home world. From the IDW comics we know that Sombra was “reformed” into a regular pony with his love, Radiant hope. What if the writers behind the G5 are smart enough to have a Sombra JUNIOR who knows about his ancestor and wants to learn about the dark magic Sombra was known for? The ancient Sombra will never allow the loss of magic as it will go against any he desires, unlimited power. But a younger Sombra born in the G5 who recently got magic, can make use of forbidden knowledge that had come into his custody?
For Sunny, a Sombra junior will be a formidable rival who will not yet be strong enough to win. He will probably only get a fraction of his ancestor’s power, similar to the handicap like Sunny who will only be a temporary alicorn. But they will both learn over time. Anyway, I do not think Sombra junior is meant to be the “end boss” in the G5.