I still don’t believe Sombra will appear in G5 only because the news came from a toy. But damn, if its real, the G5 series is gonna be really complex.
Sunny gave magic back to the non earth ponies. She’s pushing this idea that ponies don’t use their magic on each other and that there’s nothing to be afraid of. But Sombra is still technically a unicorn using unicorn magic. If G5 really does bring back Sombra, G5 is going to subvert its own narrative. That’s amazing for a kids show. That’s like anime levels of subversion.
Not likely. The movie has in a way predetermined how villains and antagonists should be in G5 S1, we saw it in the final scene when the balloon pony is startled by the sight of Mane 5. This means that Sunny will find irreconcilable ponies who could think of using magic to create division. between pony tribes. There are certainly other unicorn communities out there where unicorns are not as passive and depressed as in Bridlewood, as Equestria is a large country and the magic has probably spread to several million ponies.
I bet the pegasi are gathered under the rule of Queen Haven, not only are they deeply militaristic, disciplined and industrial, they are also very obedient under a belief that their leader should have absolute power. So it is likely that Zephyr Heights is the capital of a great empire, all the technology we see there is 100% impossible in a country with few resources and small population. But since the queen was thrown out, it is possible that there are dissatisfied pegasias that could be a problem for Sunny.
So the Mane 5 will thus find ponies that will resist the new Equestria, either to continue their prejudices - or to abuse the magic that has returned. Sombra is a little too much; after all, he managed to outmaneuver two alicorns and took his empire with him during the defeat, and then been very close to winning against the Mane 6 for the first time. It shows that he is a superb tactician and planner who is a league over Sunny, who is probably not even a permanent alicorn - and magically, his experience and expertise will make him the most powerful in the whole world.
He’s simply too much for the G5, who’s just got the magic back. If Grogar lurks in the background, it means that Sunny and her friends are in a very abrupt learning curve to learn magic, restore the Equestria, return friendship and harmony to the ponies (and later non-ponies) in all three seasons from S1 to S3. They need to face challenges in line with their growing learning ability, Sombra will simply overtake them in S1 without major problems.
Furthermore; what kind of message does it give if the wicked can survive the good over and over again? Sombra should remain buried with the Mane 6 (with the possible exception of Twilight who may appear in S3 at the latest) as the past of the G5.