Some of this is more personal opinion rather than theory- anyways.
Peach Cookie: I genuinely love this cookie and I think she'd be a fun ambush cookie. Peach Cookie could come in a more fluff update like Cherry Blossom's arrival. If Peach cookie came then Plum could come too but Peach is more popular and she was seen in the Cookie Wars teaser {a yet-to-be released game by devsisters with kingdom characters}
Timekeeper Cookie: Croissant Cookie has been present as an NPC in the game while Twizzly is running around and with how popular Timekeeper cookie is it is very likely that Timekeeper could be popping up in the future as a magic legendary
Fire Sprit, Wind Archer, Millennial Tree Cookies; all of these cookies have official VA's from the Sherbet Cookie update and since Moonlight's update was a huge hit I can see the Dev's bringing them into this soon.
Lychee, Ananas, and Lotus Dragon Cookies: Pitaya Cookie was featured in the Hollyberry Kingdom episode 12 as the final boss. If Pitaya is in the game shouldn't the other dragons make an apperence? In Cookie Oydessy Hollyberry mentions trying to get into contact with them. It is likely the Dragon Cookies will only be NPCS.
Lychee Dragon may be a bit of an odd cookie to include in the game since there more rooted in OB lore than if they were in kingdom.
Lotus Dragon would make a nice fit and they might have there own little temple hidden on the other side of a magic forcefield in the Cacao Kingdom
Ananas Dragon's style would make a nice fit in the Golden Cheese Kingdom- that would be an interesting potential antigonist.
I don't think Longan since there story is so deeply rooted in OB it wouldn't make any sense for them to show up
Sugar Glass Cookie: While mainly seen as an NPC and the Moonlight Update is over it is still possible for Sugar Glass Cookie to make her appearance. Some further explination is needed for the memory flower we see in Chapter 14. She could appear in another Oydessy-type update to continue our main story of finding the legendary cookies to fight Dark Enchantress. It is likely she would be a support that would fly over the battle feild for her skill
Lilybell Cookie: Since her first release many fans of Kingdom quickly saw the similarities between Pure Vanilla, White Lily, and Lilybell- many people calling her PureLily's child. Since Lilybell was released with the first Kingdom cookie coming to OB it would be ironic if she came to Kingdom. She'd probably come in the same update White Lily will finally become playable. She'd likely be a support or healer cookie.