Here are some cards that seem interesting:
Leavanny line: This line is a nice addition to defense-based Grass decks such as Venusaur & Celebi Gx and Rowlet & Alolan Exeggutor GX. Sewaddle has Sprout and lets you bench two Swadloon that can be evolved into Leavanny next turn to reduce 40 damage to all of your Grass Pokémon. Combined with Choice Helmet and Aether Paradise Conservation Area this constitutes serious damage reduction and combos well with healing cards such as Great Potion, Life Forest Prism and the attack of Rowlet & Alolan Exeggutor. Weakguard Energy could be used to combat the Fire weakness.
Amoongus line: Automatic sleep and poison when you play Foongus from your hand onto your bench and Slumber Forest to ensure that the opponent remains asleep.
Psyduck & Slowpoke GX, Lapras, and Misty's Request: I believe that these three cards will form a deck on their own.
Basculin: Nice bench damage in a Water Box deck.
Golisopod: Nice attack that combos well with Triple Acceleration Energy and the ability basically provides free retreat afterwards.
Tapu Koko: Nature Dive is an excellent counter to Ultra Beast decks. Some of the most relevant Ultra Beasts include Ultra Necrozma GX, Blacephalon GX, Blacephalon, Naganadel GX (both version), Naganadel, Buzzwole, Xurkitree SM10b, Nihilego, Kartana SM11a, and Pheromosa & Buzzwole GX.
Mewtwo & Mew GX: Perfection allows you to use any attack of a Pokémon GX or EX on your bench or in your discard pile. The effectiveness of this card therefore relies on it being better than the Pokémon mimiced in this manner. I can see several ways to use this card. One would be as an addition to Ultra Necrozma decks, which use Malamar to accelerate from the discard. In this deck Mewtwo & Mew GX provide an alternate attacker with a different weakness and more HP. I can also see this card as an addition in Gardevoir & Sylveon GX decks due to Miracle Twin GX as another healing option. Alternatively Mewtwo & Mew Gx could be the focus of a deck on their own. You can mimic cards such as Dragonite GX to ohko the defender and others. However, you should watch out for Power Plant and be sure to have a counter for that stadium.
Exeggcute: Nice addition to dragon Alolan Exeggutor.
Latios GX: Tag Purge is a nice defensive attack in the current meta and the free retreat is great. However, watch out for Channelor.
Aegislash: Undying Sword is a nice ability. Not sure where to include this card at the moment but the ability is appealing.
Necrozma and Recycle Energy: This could be a new core attacker in Malamar decks combined with Recycle Energy to ensure maximum damage output. it may even make a non-GX Malamar variant possible.
Espeon & Deoxys GX: Nice addition to Malamar decks with a stronger focus on spread. Cross divide can be powered by Double Colorless Energy in case it remains in format too.
Jirachi GX: While being a weak bench sitter the ability to cancel Psychic weakness on any Pokémon is relevant in this format. I can see this as an addition to selected Tag Team decks but I am reluctant to use it in Malamar decks because it can easily turn into a cheap two prize cards for the opponent.
Steelix and Channelor: Land Stream accelerates to itself and Giga Shake hits numbers combined with Material arts Dojo and Diancie Prism Star. You could turn off the detrimental side effect of Giga Shake with Channelor similar to prior used of Pokémon Range.
Aerodactyl GX: Prehistoric Wind is essentially Wonder Labyrinth on a Pokémon and I could see this work well Pokémon Research Lab if Double Colorless Energy remains in format.
Terrakion: Provided that Double Colorless Energy remains in format Cave King could be a real good attack when combined with Spiritomb and Material Arts Dojo.
Mega Sableye & Tyranitar GX: The side effect of the attack is nice but the high energy costs turns me off here. I know this is ment to work with Morgan but I do not like that engine that much.
Sableye: Eight damage counters when hit by the opponent is not bad at all for a set-up Pokémon that can utilize Black Market.
Umbreon & Darkrai GX: I do like Black Lance and Dead Moon but the deck could turn out to be a little clumky and slow similar to Greninja & Zoroark GX.
Weavile GX: Shadow Connection is nice but we need to keep in mind that this is not energy acceleration per se. You could turn it into an energy acceleration engine with Naganadel but this will make the deck a little more clunky.
Whimsicott: Great tech addition to Fairy deck to search out specific cards such as Tiple acceleration energy in a Whimsicott GX deck.
Dedenne: Nice set-up Pokémon that allos you to play Pokémon Fan Club once Ultra ball and Nest Ball rotated.
Dragonite line: Nice energy acceleration engine that combos with Dragonite GX.
Dragonite GX: Sky Judgement is the main attraction here and it combs well with Triple Acceleration Energy. Being a colorless attack it can be used in various different decks in principle and also constitutes a great target to mimic for Mewtwo & Mew GX.
Komala: Nice addition to Slumber Forest decks.
Cherrish Ball: Universal search that puts Pokémon GX into your hand. You can search out Basic Pokémon GX, and evolved Pokémon GX alike. What#s not to like about this card?
Great Potion: Could see this card become relevant in specific decks such as Lucario & Melmetal and defensive Grass decks.
Stadium Navigator: Great search card in decks that have the space for it and benefit a lot from a certain stadium card.
Giant Bomb: Nice alternative to Spell Tag against decks that cannot hit for less than 180 damage.
Misty's Request: While being part of a deck on it's own I can also see this card in decks that revolve around energy acceleration with Morgan.
Channeler: Nice counter to decks that attempt to lock the opponent with effects of attacks. Channelor is in effect a Pokémon Ranger reprint.
Blue's Strategy: Nice addition to out arsenal of draw supporters. While not a stable four-off on it's own it can be a two-off in certain decks.
Hapu: A more aggressive and universal version of Bill's Analysis.
Slumber Forest: Great addition to status lock decks.
Pokémon Research Lab: Nice means to get out Fossil Pokémon fast in the early game. Both versions of Aerodactyl seem to benefit the most from this stadium. In Gengar and Mimikyu GX with Omastar I still prefer Unidentified Fossil since you cannot afford to loose a turn in this deck.
Night City: Free retreat is always nice and helps to make room in some Darkness decks.
Weakguard Energy: I believe that this card is underrated since it could work well in Tag Team decks that are hurt by having a more commonly used weakness. Also note that in this deck you can move it around with Tag Switch.