What made the Battle Frontier in Emerald interesting for me is the differences in the battle rules between the different facilities. The Tower was basically just battles, reincarnated in SM as the Battle Tree. And you had the Dome for 2v2 with team preview, which was later transferred to competitive. The Arena for quick battles with judging, based on damage. The Palace for a bit of randomness and having natures determine the battle style. And the Pyramid was pretty much survival mode. I wasn't too fond of the Pike or the Factory, but overall there was a lot to do there, and you needed to adapt your teams to the facilities.
The problem with the Battle Spot is that although it works well, it's limited in the options it has. It doesn't have triple or rotation battles, it doesn't offer 6v6 or 4v4 singles as options, nor does it offer 6v6 doubles. Those formats require different team layouts compared to 3v3 singles and 4v4 doubles. And personally, I'd also like an option for allowing legendaries like Solgaleo and Kyogre. They're now basically useless to train competitively. The link battles just don't work well enough, most people visiting Plaza don't have a Battle Team ready to use, and even if they accept a challenge they occasionally just use their party with stuff like Charmander or Rowlet in it. And having to go to third-party websites to arrange link battles is just so much less convenient than hopping into the Battle Spot, it's such a hassle to go fiddling around with friend codes and time zones and everything.
