I think when people ask for a new Battle Frontier, they want an updated version of it, with better facilities, better prizes for beating the Bosses and rules that are easier to understand. Maybe attach some story and lore to it aswell and allow you to unlock some cutscenes by defeating the Brains, or something like that, so it doesn't feel like a shallow experience.
Like, how fun XY's postgame would have been if it had more facilities with each Boss using a Mega and giving you their Stones unlocking new lore about Mega Evolutions and how they came to existence once you collected all of them?
Anyway, the thing i like about the Battle Frontier is the variety. It gets tiring seeing the same Battle Tower copycat over and over again. Even something as simple as tweaking the rules of the battle a little bit can make things a lot more fun. The Battle Arcade for example is really fun and all they had to do was add a Roulette.
But if we were to only have copies of previous facilities, i wish they would bring more of the fan favorites. I think it would be nice to have a facility like the Maison, a facility like the PWT, a facility like the Battle Factory and a facility that revolves around exploring, like the Battle Pyramid or the White Treehollow.