I've heard that one and it feels like a pretty good option to shake the fewest cans of worms. I think. We know there's an Equestria--Earth portal (that was never explained) so other dimensions are totally possible! I've never heard of a multi-verse theory that stops at two. :3
I think the premise of all magic being gone is interesting (and in no way am I biased because my Shadowtrot setting went through that scenario before it whipped back with a vengeance, lol). If nothing else, I'm curious to see what kind of villains they might throw in G5.
Having seen some episodes recently, I would say a bit of both. I really liked that Trixie/Starlight played off well together, but it does get me thinking that maybe Starlight wasn't necessary at all when we had Trixie. And between the two Trixie had the better character arc stuff going on. :p