You're definitley right about Malamar improving for SM-on format. No Parallel City to limit space, that's one thing (though I'll add in through my personal experience that I was fine having 2 Malamars and 2 other pokemon under Parallel City when it was legal while playing online as 2 Malamars were enough. Infact it might've been better to be "Parallel'd" when you get rid of targets like Tapu Lele-GX and then get rid of it later by Field Blower to reset which P0kemon take your bench space). More importantly, no BKP Garbodor / BKP Greninja to lock abilities limiting Slaking as the only option (at the timing of this post) to have universal ability lock for your opponent, as Glaceon-GX and Alolan Muk don't do anything against it. Oh and as a bonus, Shining Lugia is slowly becoming it's new partner and Shrine of Punishment exists now.
I think what's gonna stop it from being one step above tier 1 is Zoroark-GX. Most Malamar vairants have a bad match-up against this card when Zoroark (esspecially when paired with Lycanroc) can bring up Malamars to the active and get a clean OHKO on them ruining some set up, in addition to when half of the stuff Malamar generally charges to doesn't OHKO Zoroark, examples being Necrozma-GX and Dawn Wings Necrozma-GX. Zoroark also has just better consistency. Sure you can set up the Malamars with more search power thanks to Mysterious Treasure, but past that you're relying on draw supporters to get your consistency going, while Zoroark's Trade can get you that and way more for the whole game he's in.
At least Malamar essentially has access to Marshadow-GX, and it did prove to be quite effective against Zoroark decks when he was played, so I guess that depends on how popular Marshadow-GX becomes as a tech for the deck, as Marshadow isn't too good against much else with his rather annoying 150HP. Zoroark's can OHKO it with a full bench and Choice Band and with the lack of Field Blower being played now that it's no longer mandatory for BKP Garbodor, Zoroark decks (and probably some other decks as well) might even run Weakness Policy to take advantage of the ladder. We'll see what happens then.
Malamar decks for Worlds 2019? Hold on there, we still got like 3 or 4 sets before Worlds 2019 even happens, so that could change a lot of things. Maybe in those upcoming sets we do have a good universal ability lock card or more hand disruption that could greatly affect Malamar decks. Silvally-GX is still a Stage 1 meaning you would have to run more cards in your deck just to get his ability going. While I have seen him in some interesting decks, he's no Float Stone, and he esspecially won't go in Buzzwole decks.