I am forever sadden by the fact we never got to have atleast one battle with Cogita or see what Pokemon she might have on a team. Given I have so many thoughts on her, I wanted to share what I think her team could be. This list is in no specific order:
Gastrodon, as Cynthia had one on her team in the original Diamond and Pearl before it got replaced by Togekiss in Platinum. It appears again on Cynthia's team in BDSP, Cynthia has it on one of her teams at the BDSP Battle tower and she can get a Sync Pair with it once reaching Trainer Lodge Friendship Level 100 with Cynthia. It feels like it would make a lot of sense. I personally lean towards her having a West Sea on to match Cynthia, although given how the Ancient Retreat is closer on the map to the East side of the region, so you could argue it could be the East Sea Gastrodon. Either way it makes sense for her to atleast have a Gastrodon.
Porygon-Z, despite Porygon-Z, and the line as a whole showing up in space-time distortions, I could see her obtaining one while doing a personal study of the distortions by wondering into one when they first started showing up in Hisui. Plus, the space-time distortions Porygon-Z shows up, are the ones in the Crimson Mirelands, a place near the Ancient Retreat.This Porygon-Z would also be a reference to the one Cynthia has during her Rematch (after Stark Mountain) in BDSP.
Sylveon, Some of these next thoughts might be a stretch, starting with this one. The way Cogita talks about Enamorus and helps the player character obtain it, reads to me that they were friends and that Cogita didn't necessary caught her (The only thing I see that happening would be as a future Sync Pair in Pokemon Masters EX). So when looking at what other Pokemon Cynthia has used, I remembered she uses a Glaceon on her in Pokémon Black 2 and White 2 both in her Undella Town battle and at the Pokémon World Tournament. Not wanting to give Cogita the same Pokemon that is Irida's ace, I landed on giving her Sylveon. As a connection to Cynthia using an Eeveeluiton, Cynthia and Volo having a fairy type in Togepi/Togekiss and Enamorus also being a fairy type.
Hisuian Braviary, Hisui has a lot of ties to Unova, it always kinda baffles me. As I kinda forget just how much there is and you know who has a Braviary on a team in Unova, Cynthia. Giving Cogita a Hisuian Braviary just fits. She has a connection to Unova thanks to forces of natures and now a Braviary.
Hisuian Goodra, this might be personal bias as I adore Hisuian Goodra with my heart and soul, just like I adore Cogita. However, Cynthia's bloodline seems to have a choke hold connection to both the fairy and dragon types. Hell Cynthia has Sync Pair's with 3 different dragon types; Garchomp, Kommo-o and Giratina. Volo gets to use Garchomp and Giratina TWICE (The charming bastard). Cogita deserves one dragon and I think that should be a Hisuian Goodra. Plus you can first find Goomy and Sliggoo in the Crimson Mirelands, near the Ancient Retreat. It feels right to me.
Mismagius, there is no reason for this. There is nothing I can stack as evidence to help booster this idea. This pick is 100% on vibes alone, baby.
If anyone else has thoughts on this, I would like to hear them.