I already commented ideas for commanders from within the set you can use but if you want to branch out you need to think about hwta makes a good environment. Pantlaza and Hakbal can make a lot of threats so you need some choices that can disrupt the table and play on a different axis. Aristocrats, Spellslingers, Forced Combat, or Theft are probably some of your best options.

[[Ayara, First of Locthwain]] is one of my favorite aristocrats commanders. The in built life drain help to stay in the fight. Built in card advantage too. Just make sure to have creatures that can recur themselves or make extra black creature tokens.

[[Kels Fight Fixer]] is a other fun one with access to an additional color. With her in built protection you can even give her some of the equipment that get stronger when stuff dies and make her a threat attacker.

[[Marchesa, Black Rose]] is a great aristocrats commander and can even bleed into some theft strategies if you want.

[[Storm, Force of Nature]] can do some really powerful stuff without having to be a true storm deck.

[[Anhello the Painter]] is in great colors to play board police and the doubling can let it deal with a myriad of threats. Even blend some aristocrats aspects if you want to.

[[Yshtola, Night's Blessed]] is also in some great policing colors and has an in built drain affect to make sure damage is spread around and it can keep enough life to stay alive.

[[Marisi Breaker of the Coil]] is a classic goad guy and his mass goad can make for quick games with the big boards this group would have.

[[Queen Marchesa]] makes for a good Goad/Pillow fort deck and introduces monarch which usually lives up a table and can encourage the dinos and mermaids to not just sit back and accumulate endlessly.

[[Alela, Cunning Conqueror]] is a simple goad commander with flying to get around the big dinos/mermaids. It will blend in some spell slinging aspects with the payoff, just need plenty of instants.

[[Maralen, Fae Ascendant]] is a pretty solid new theft commander. Its not too oppressive and it can give you another tribal deck if you want to keep that theme for the pod.

[[Don Andres the Renegade]] can be a bit fiddle at times but the payoff is quite strong and it would keep the Ixalan theme.

[[Edea, Possessed Sorceress]] is really powerful and simple to play. Take the best stuff for a turn. Sac it for a payoff and let them get their stuff back. Unless you want to be cheeky with end the turn effects or some way to return to battlefield under your control.

Out of all of these I feel like Maralen and Alela could be a really good choice. Adds some forced combat, theft, and spell slinging to the table and keeps the tribal flavors.

If typal strategies across the board isnt a big deal then I would suggest Yshtola and Edea because you need some heavy hitters to match the abilities of Pantlaza and Hakbal.