Split this between real-life (adjacent) versus in-game. Answers differently due to different priorities.
Real-life:
Poses a more pressing question first: what is the plan? Why would you even go? What are you going to gravitate towards?
Would not be Pokemon battling. Guesses something more observational. Leans towards Pokemon research. Leaves the door open for Pokemon breeding/care. Overlaps a bit, probably.
- Brock (from Red/Blue with the anime in mind). Knew more about handling Pokemon than most. (Stopped watching the anime after Generation 1. Could have introduced more.) Seems like a source of knowledge.
- Sonia (from Sword/Shield). Makes sense to bring a Pokemon professor along. Mentioned liking to camp too. Bets on far too much of that during this journey.
- Lillie (from Sun/Moon). Matches their personality the closest (at least, among major characters). May be good to bring another student-type along.
Considered the various makers of Pokemon technology (Bill, Lanette, Bebe, Amanita, Cassius, Sophocles, Molayne, and maybe others?). Doubted useful contributions from them during a journey. Seeks them out afterwards, perhaps.
In a game:
Aims for good rivals and pleasant conversation.
- Lana (from Sun/Moon). Seems fun to bring along. Livens up some conversations.
- Marnie (from Sword/Shield). Felt humble, despite the huge fanbase.
- Piers (from Sword/Shield). Respects the mindset of battling how you want. Applies to many gym leaders, admittedly. Takes it a bit further with refusing to Dynamax. Did the same in the first Sword playthrough too (minus Max Raid situations).